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UNITED STATES OF AMERICA. 



i ELEMENTS **^z&- S 

OF 

ELECTRICITY, MAGNETISM, 

AND 

ELECTRO-DYNAMICS, 

EMBRACING 

THE LATE DISCOVERIES AND IMPROVEMENTS, 

DIGESTED INTO THE FORM OF A TREATISE ; 

BEING 

THE SECOJVD PART 

OF 

A COURSE OF NATURAL PHILOSOPHY, 

COMPILED 

FOR THE USE OF THE STUDENTS OF THE UNIVERSITY 

AT 

CAMBRIDGE, NEW ENGLAND. 



By JOHN FARRAR, LL.D. 



BOSTON: 
HILL1ARD, GRAY. AND COMPANY. 

1839. 




Entered according to Act of Congress, in the year 1839, by 

Hilliard, Gray, and Company, 

in the Clerk's office of the District Court for the District of Massachusetts. 

/ 



CAMBRIDGE: 

FOLSOM, WELLS, AND THURSTON, 

PRINTERS TO THE UNIVERSITY. 



"b 



ADVERTISEMENT. 



The articles Electricity and Magnetism, in this vol- 
ume were selected from Biot's Precis Elementaire de 
Physique, and translated with only such alterations as 
were deemed necessary in order to adapt the work 
to the English reader. There being no edition of Biot's 
treatise sufficiently recent to correspond to the present 
improved state of Electro -dynamics, the portion of the 
volume relating to this subject was taken from Des- 
pretz's Traite Elementaire de Physique, fourth edition, 
published at Paris in 1836. 

J 837. 



CONTENTS. 



ELECTRICITY. 



General Phenomena of Electrical Attraction and Repulsion ; 
Conductors, and Non- Conductors ; tioo Kinds of Elec- 
tricity ......... 1 

Of the Laws which govern the apparent Attraction and Re- 
pulsion of electrified Bodies . . . . .13 

Of the Laws according to which Electricity is dissipated by 
the Contact of the Air, and along the Supports which 
retain it imperfectly ....... 22 

Of Electricity in a State of Equilibrium in insulated Con- 
ducting Bodies ....... 30 

Of Combined Electricities, and their Action at a Distance 39 
Theory of the Motions produced in Bodies by Electrical 
Attraction and Repulsion ...... 58 

Of the Construction of Electrical Machines . . .67 

Of Electroscopes . . . . . . . .75 

Of the Condenser ....... 80 

Of the Ehctrophorus ...,., . 90 

Of the Leyden Jar ....... 94 

Of the Electric Battery . . . . . . . 97 

Of the Electric Pile and of the Phenomena presented by 

Crystals capable of being electrified by Heat . .100 

Mechanical and Chemical Effects produced by the repulsive 

Force of accumulated Electricity . . . .103 

Of Atmospherical Electricity and Lightning Rods . 106 

Of Electric Light 118 

Of the different Methods of developing Electricity . 121 

Of the Develop: ment of Electricity by simple Contact . 126 
Theory of the Voltaic Apparatus, on the Supposition that 

its conducting Power is perfect . . •'■■ . .140 

Chemical Effects of the Voltaic Apparatus . . . 149 



vi Contents. 

Examination of the Changes ivhich take place in the Voltaic 
Apparatus by its Action upon itself. — Effects ivhich 
hence result in its Electrical State . . . .171 

Of Secondary Piles . . . . . .184 

On the unequal Resistance which the tioo Electricities, when 
very weak, meet with in traversing different Bodies . 188 

MAGNETISM. 

General Phenomena of Magnetic Attraction and Repulsion 193 
General Considerations respecting the Developement of Mag- 
netism. Resemblance to the Electric Pile . . . 206 
Determination and Measure of the Directive Force exerted 

by the Terrestrial Globe upon Magnetized Needles . 210 
Of the Different Methods of Magnetizing . . .225 

General Distribution of Free Magnetism in Wires mag- 
netized by the Method of Double- Touch . — Laws of 
Magnetic Attraction and Repulsion . . . .246 

Of the Intensity of Free Magnetism in each Point of a 
Needle magnetized to Saturation by the Method of 

Double- Touch 251 

Of the best Forms of Compass Needles . . . 257 

Of the Action of Magnets on other Natural Substances . 260 
Of the Laws of Terrestrial Magnetism in different Latitudes 262 
Aurora Borealis ........ 278 

Periodical Variation of the Needle .... 289 

Practical Instructions as to the Method of observing the 

Elements of Terrestrial Magnetism .... 292 

ELECTRO-DYNAMICS. 

Disturbance of the Magnetic Needle by the Electric Current 305 
Attraction and Repulsion of Electrical Currents . . 308 

Closed Currents and Solenoids . . , , . 314 

Reciprocal Action of Solenoids . . . .318 

Action of Solenoids upon Conducting Wires . . 321 

Law respecting the Intensity of Currents .... 325 
Mutual Actions of Magnets and Currents . . . 326 

Continued Revolution of a Magnet by a Current about a 
Line parallel to its Axis ...... 327 






Contents. vii 

Rotation of a Magnet about its Axis by the Action of a 

Current 329 

Rotation of a Voltaic Conductor upon its Axis by Means of 

a Magnet 330 

Rotation of Mercury by a Magnet .... 330 

Action of the Earth upon Voltaic Conductors . . .331 

Astatic Conductors ....... 333 

Of Magnetizing by Electric Currents . . . . 334 

Description of an Apparatus by Means of which all the 
Experiments relating to Electro-Dynamics may be per- 
formed 338 

Experiments upon the Mutual Action of two Rectilinear 

Currents, making any Angle with each Other . . 342 
Experiments on Continued Rotation produced by a fixed Con- 
ductor , by the Action of the Earth, or by that of Currents, 
which are established in the acid Solution, in which the 
movable Conductors are immersed .... 344 
Experiments upon Helixes and Solenoids. . . .346 
Of the Multiplier 346 

NOTES. 

I. The Torsion Balance 349 

II. The best Form of Lightning Rods, extracted from 

a Paper of M. Gay-Lussac . . . .351 

III. Hare's Calorimotor and Deflagrator . . . 356 

IV » Electrical Fishes 358 

V. Declination of the Magnetic Needle at London, from 

the Time it was first observed .... 360 

VI. Dip or Inclination of the Needle at London . 361 
VII. Diurnal Variation of the Magnetic Needle . . 362 

VIII. Influence of Magnetism on the Rates of Chronometers 363 
IX. Local Attraction and Barlow's Correcting Plate . 363 
X. Theory of Magnetism, by M. Poisson . . 371 

XI. Effects of Temperature on the Magnetic Forces . 373 
XII. Diurnal Variation of the Terrestrial Magnetic In- 
tensity ...... . 374 

XIII. Magnetism in Motion . , , ,.■ . 374 



ERRATA. 

Page 68, line 30, for 100°, read 212°. 
" 129, " 2, " Fig. 59, read Fig. 51. 
" 201, " 17, after property, read Manganese, when reduced to a low temperature, is said to 

possess the same property. (Pouillet.) 
" 260, " 14, after about, insert one fiftieth of 
" 261, " 3, for develope, read developes 
" 265, " 1 Sf 2, for an easterly, read a westerly 

" 268, " 29, after Paris; read See Note on the Declination of the Magnetic Needle at London. 
" 269, " 9, for Gilpins read Gilpin 
« « " ]2, after 21'. read See Note on the Dip or Inclination of the Magnetic Needle at 

London. 
" 282, " 25, for form, read from 
" 289, " 34, « 112. " 122. 
" 303, " 23 <J" 24, (in some copies) for under, read undergo 



ELECTRICITY. 



General Phenomena of Electrical Attraction and Repulsion ; Con- 
ductors and JVon- Conductors ; two Kinds of Electricity, 

] . The properties which we have hitherto discovered in bod- 
ies seem to be inherent in them, and permanently attached to the 
matter of which they are composed. Thus heavy bodies can 
not be deprived of their gravity, nor their particles lose the prop- 
erty of mutual attraction. 

We come now to consider certain transient states, or modifi- 
cations, of which bodies are susceptible, and which are the more 
remarkable, since, without adding to their particles, or taking 
from them, any tangible or ponderable principle, they ar? not- 
withstanding attended with very powerful mechanical effects, 
which may be seen in the motion of material bodies. 

For example, if we take a stick of sealing-wax, or a glass 
tube, or a piece of amber, which has been for a long time un- 
touched, and bring it near some small pieces of paper, chaff, 
or other light substance, no impression is produced ; but if we 
first rub lightly and briskly, the glass tube, the sealing wax, or 
the \mber, with a piece of dry woollen cloth, or cat skin, upon its 
being brought near either of the light substances above mentioned, 
a strong attraction will be manifest. We have here a new prop- 
erty or faculty developed by friction, and which did not previ- 
ously exist. This property has been called electricity, from the 
Greek word 7]Xaxrgov, which signifies amber, this being the sub- 
stance in which it was first observed. 

Several centuries passed without any thing being known be- 
yond the simple fact just stated ; but for the last sixty years the 
E. £ M. 1 



2 Electricity. 

phenomena have been more carefully examined, and have thus 
led to the discovery of many important results, which together 
form one of the most interesting parts of natural philosophy. 

The first step to be taken, is to study carefully the funda- 
mental phenomenon above described, and to examine all the 
various circumstances under which it presents itself. By rub- 
bing tubes of glass, sulphur, or sealing wax, of considerable size, 
an inch in diameter, for example, and a foot long, light bodies 
are attracted from a distance ; and they are seen to rush with 
great rapidity against the electrified tube. Some adhere to it, 
others, upon coming in contact with it, are immediately repelled. 
If the tube be brought near the hand or the face, at a certain 
distance a sensation is felt similar to that produced by a cob- 
web; and if it be touched with the finger or a metallic ball, a 
spark darts with a crackling noise from the tube to the body 
presented to it. When the experiment is performed in the dark, 
this spark becomes vivid, and we constantly observe a bluish 
light following the rubber as it passes along the tube. The ef- 
fect may be still further increased, by substituting for the tube a 
large globe, or cylinder, or plate, fixed between two cushions, 
and made to turn by means of a handle. This apparatus is 
called an electrical machine. It is ordinarily accompanied with 
other appendages, which render its effects much more certain 
and intense ; of these we shall speak hereafter, when we have 
treated of the theoretical principles on which they depend. In 
the mean time, the apparatus, such as we have described it, is 
sufficient to establish the fundamental phenomena which we 
have stated. 

It may now be asked, what is the nature of the principle un- 
der consideration ? how does it exist in bodies? how is its action 
developed by friction ? These questions we are unable to an- 
swer ; but whatever be the cause of the phenomena in question, 
to avoid circumlocution we shall call it electricity, just as we give 
the name of caloric to the unknown principle of heat. 

All vitreous and resinous substances are capable of exhibit- 
ing the phenomena above mentioned in different degrees. Silks 
also answer the same purpose ; but if we take a metallic tube and 
rub it with a cat skin or a piece of woollen cloth, it will pre- 
sent no luminious appearance, it will excite no sensation, nor 
manifest any disposition to attract light bodies. 



Conductors and Non-Conductors, 3 

2. If, however, instead of taking the metallic tube in the hand, 
we hold it by means of a tube of glass or resin, and rub it as be- 
fore without its touching any other substance but the rubber, it 
will acquire all the electric properties of glass or amber. The 
same phenomena occur, also, if instead of the glass or resinous 
handle, we make use of a silk holder, consisting of several thick- 
nesses, or if we suspend the metallic tube by means of silk cords. 
The electric properties will continue, however, only while the 
tube has no other communication with surrounding bodies ; for 
if we touch it with the finger or with another piece of metal, 
all signs of electricity instantly vanish. 

It is evident from these experiments, that if the metal did 
not at first acquire electric properties by friction, it was not 
because it was incapable of receiving them; but because it 
could not retain them ; for when it is made to possess them, it 
may be deprived of them immediately by touching it with the 
finger or with another piece of metal. Thus when it is held in 
the hand and rubbed, the electricity is dissipated as fast as it 
is developed. We must not, therefore, be surprised that no ef- 
fect is produced. But the electricity becomes sensible when the 
metal is suspended in the air by means of glass, resin, or silk. 
We infer, therefore, that these substances resist the passage of 
electricity ; we know, moreover, directly, that electricity does not 
readily pass along a silk riband, a glass tube, or a stick of resin ; 
for when one of these substances is rendered electrical by fric- 
tion, if we touch one part, we deprive this of its electric prop- 
erties without affecting the rest. On this account, we can elec- 
trify bodies of the above description by friction, while holding 
them in the hand, but not those of a metallic nature. 

We are thus led to distinguish natural bodies into two great 
classes, according as they do or do not transmit the electric 
principle, and which are hence called conductors and non-conduc- 
tors ; the latter are also called insulating bodies, because, when 
they are employed as supports, they serve to cut off all commt£" 
nication between a conducting body and other conductors which 
might deprive it of its electricity, t 



t Formerly, non-conducting bodies were called electrics per se, or 
idio-electrics, that is, self-electrical ; and conductors were called ant- 



4 Electricity, 

The atmosphere is evidently of the class of non-conducting 
bodies ; since, if it afforded a free passage to electricity, no 
body surrounded by it could exhibit durable electrical phenom- 
ena. Now a tube of glass or resin, being rubbed, preserves its 
electric properties for a considerable time, although immersed 
in this fluid. 

Water, on the other hand, is a conductor ; for if we moisten 
with this liquid or only with its vapour, a tube of glass or resin, 
electrified by friction, it immediately loses all its virtue. Thus 
the aqueous vapour suspended in the air impairs the insulating 
properties of this fluid, and it is for this reason that electrical 
experiments succeed best in cold and dry weather, because then 
there is less vapour contained in the atmosphere. 

This difference in the disposition of different bodies to retain 
and transmit electricity, was first made known by Grey. He 
owed the discovery to accident, but to an accident of which he 
well knew how to avail himself. 

There is no constant relation between the state of bodies and 
their conducting power. Among solid bodies, the metals trans- 
mit electricity readily, dry gums and resins scarcely transmit it 
at all. Almost all liquids are good conductors ; oil, however, 
is a very imperfect conductor. Wax and tallow, when cold, con- 
duct badly ; when melted, they conduct well. The power of 
conducting electricity is observed in the most opposite states ; 
for example, in the flame of alcohol and in ice. The tempera- 
ture of bodies seems to have no sensible influence on the elec- 
tric sparks which proceed from them. Those which proceed 
from ice are not cold, and those which proceed from red hot 
iron, do not appear to have their heat increased. 

The air and dry gases, besides their insulating property, 
seem also to have the faculty of confining electricity upon the 
surface of bodies by the force of pressure. For, if we place 
under the receiver of an air-pump, a conducting body electrified 
and insulated upon supporters of glass or resin, this body, when 

lectrics, or non-electrics, because it was believed that the first only 
could be electrified by friction. This was an error. All bodies become 
electrical by being rubbed, but all are not capable of retaining the 
electricity thus developed, without being insulated. 



Conductors and Non-Conductors. 5 

the air is rarified to a certain degree, loses all its electricity, which 
shoots off with a bluish light towards the conducting bodies by 
which it may communicate with the ground. If we place under 
the same circumstances, a non-conducting body, a stick of sealing 
wax, for example, electrified by friction, the electricity aban- 
dons it also as soon as a vacuum is produced, but more slowly 
than in the case of a conducting body, and with a sensible inter- 
val of time. These phenomena, therefore, seem to indicate that 
the electricity is retained upon the surface of conducting bodies 
only by the pressure of the air ; and that at the surface of noncon- 
ducting bodies, as dry glass and resin, it is retained by this pres- 
sure, joined to the difficulty which it meets with in disengaging 
itself from their particles. 

The conducting property of the metals is advantageously 
employed to facilitate the operation of the electrical machine. 
We suspend by silk cords, or place upon glass cylinders, a me- 
tallic bar one side or one end of which is brought very near 
the globe or plate, which is electrified by friction. Then, as the 
electricity is developed, it passes to this insulated metallic con- 
ductor, and is retained there. If we touch this prime conduc- 
tor, as it is called, with another metallic bar insulated in the same 
way, this second bar becomes electrical also, and the electricity 
may thus be transferred wherever we please. It is of little im- 
portance at what point we touch the prime conductor ; it will 
give its electricity from any part. If we attach to it a metallic 
wire of any length, as a thousand yards, for example, this wire will 
also become almost instantaneously electrical through its whole 
extent, provided it is equally insulated in every part. We may 
also continue the communication through portions of water, in 
a fluid state, contained and insulated in vessels of glass. These 
are the consequences and the proofs of the free passage which 
conducting bodies offer to electricity. 

To insure success in our experiments, it is necessary that 
the silk cords or glass tubes which serve to insulate conductors, 
should be very dry; otherwise the electric properties grow 
weaker and weaker, and soon cease entirely. Very fine 
dry silk thread forms an excellent insulator for light bodies. 
If we suspend to a thread of this description, a small ball of el- 
der pith, which is extremely light and a good conductor at the 



6 Electricity. 

same time, we shall have a very simple and convenient instru- 
ment for studying the theory of electricity. This little pendu- 
lous body is usually attached to a moveable stand, as in fig- 
ure 1. 

3. If the pith ball is brought in contact with an electrified 
glass tube, and is then separated without being touched, it will 
be found to have acquired electric properties. It will attract 
chaff", dust, and other light substances which are presented to it. 
It will be drawn toward the hand if placed near it ; in a word, 
it has been electrified by communication. 

When the air is dry, these properties will continue a con- 
siderable time, provided the ball remain unconnected with any 
conducting substance ; but if it be touched, it will immediately 
return to its natural state, and its electricity disappear. 

4. Here, as in the case of the electrified conductor, it may 
be asked what becomes of the electricity, and why does it pro- 
duce no effect ? The following experiment will enable us to an- 
swer these questions. 

If, instead of touching the ball with the finger, we touch it 
with another ball, eighty or a hundred times as large, suspended 
in the same way, we shall find that the first has lost its electric 
virtue almost as completely as if it had been touched with the 
finger. We thus perceive that a given quantity of electricity 
loses in intensity by being distributed over a larger surface ; for 
the interior of the balls has no effect, and whether they be emp- 
ty or full, the phenomenon takes place in the same way. After 
this, it will be readily understood that the little ball loses its elec- 
tric virtue, by dividing it with the human body and the immense 
mass of the earth, which are conducting bodies communicating 
with it. It is on this account that we often call the earth the 
common reservoir of electricity. 

5. Let us now examine more carefully what takes place 
when we bring the pith ball toward the electrified tube. At 
first it is attracted by the tube, and adheres to its surface ; but 
after a short interval, just sufficient for the electricity of the tube 
to be communicated, it is repelled and seems to fly oft as long as 
it preserves its electricity. By bringing the tube, however, very 
suddenly near the ball, we sometimes make the ball return, and 
thus change its repulsion into attraction ; this is a compound 



Two Kinds of Electricity. 7 

phenomenon, the cause of which we shall explain hereafter ; but 
confining ourselves for the present to what takes place when the 
tube is presented to the ball from a distance, for the purpose of 
foretelling its motions after a part of the electricity has been 
communicated to it, we see that it always begins with flying from 
the tube. Hence we derive this important conclusion, that with 
the exception of certain particular cases, the cause of which re- 
mains to be explained, bodies electrified by communication, 
mutually repel each other. 

It would at first seem that the preceding experiment did not 
fully authorize this conclusion. We indeed see that the ball flies' 
from the tube, whose electricity it has shared, but it does not ap- 
pear that the tube flies from the ball. The sole cause of this how- 
ever is, that it is too heavy. The ball only is displaced, not being 
sufficient to move the tube ; but to present the subject fairly, we 
take two equal pith balls, and attach them to the two extremities of 
a linen thread, which is a conductor ; we next suspend this thread 
from its middle point by a thread of silk, as represented in fig- 
ure 2 ; then the two balls will communicate by the linen thread 
and the silk thread will insulate them both. Now if w r e touch 
the two balls, or only one of them with an electrified tube, 
we shall see that they will not only fly from the tube whose 
electricity they have shared, but from each other, and the two 
parts of the thread will diverge, as represented in figure 3. Fig. 3. 

6. The repulsion of the little electrified ball takes place 
equally, whatever be the nature of the tube which is employed 
to give it electricity, provided that it be always the same tube 
that is afterwards presented to it. But if after having communi- 
cated to it the electricity of a glass tube rubbed with woollen, we 
bring toward it a tube of sulphur or resin, rubbed with the same 
substance, instead of flying from this second tube, it will approach 
it, and rush with more force than it would do, if it had not been 
previously electrified. The same thing happens if we begin by 
electrifying the ball with the resinous tube, and afterward bring 
toward it the tube of glass ; attraction takes place equally in 
each case. 

We find, therefore, that when a body has been electrified 
and insulated like the little pendulum above referred to, other 
electrified bodies which are brought near it, do not all act upon 



C Electricity. 

it in tliC some manner, since some repel and others attract it. 
We are hence led to distinguish electricity into two kinds, the 
one analogous to that produced by glass, when rubbed with 
woollen, and which we shall call vitreous electricity ; the other sim- 
ilar to that produced by resin, rubbed also with woollen, and 
which we shall call resinous electricity. This important distinc- 
tion was first observed by Dufay. 

All the phenomena, then, of attraction and repulsion which 
we have thus far observed may be expressed by this very sim- 
ple law ; bodies charged with electricity of the same kind mutually 
repel each other ; but when they are charged with different electrici- 
ties they attract each other. 

Although this proposition seems to be purely the enunciation 
of facts, yet we must not attach to it the idea of absolute reali- 
ty ; for motions perfectly similar to those presented by electri- 
fied bodies may be produced without any real attraction or re- 
pulsion among the material particles. For example, imagine a 
glass vessel AB filled with a heavy fluid, as water or mercury, 

Fig. 4. and suspended vertically by a cord from a fixed point S. If 
this vessel be not touched, it will remain at rest in virtue of the 
laws of equilibrium, and the fluid which it contains will give it 
no horizontal motion, because the lateral pressures, exerted at 

451. the same depth in the opposite directions AB, BA, are equal to 
each other. But suppose that with a burning mirror MM we 
direct a cone of light upon the point A, and thus cause a small 
hole in the side of the vessel at this point ; then, the fluid flowing 
freely through this hole, the pressure in the direction BA will 
become nothing, and the pressure in the direction AB having 
nothing to counterbalance it, the vessel will recede from the 
mirror as if a repulsive force were exerted between them. On 
the contrary, if the focus of the cone were directed to the point 
B through the matter of the vessel, the fluid being supposed to 
to be transparent, the vessel will approach the mirror as if it 
were urged by an attractive force. Still there is no absolute at- 
traction or repulsion ; the motion observed is the simple efiect of 
the proper hydrostatic pressure of the fluid contained in the ves- 
sel AB. Now this ought not only to put us on our guard against 
admitting the idea of a real attraction or repulsion between the 
material particles of electrified bodies ; but we shall see by and 



Two Kinds of Electricity. 9 

by, that the motions of these bodies are produced by a precisely 
similar mechanical action ; for their material particles, although 
electrified, do not acquire any real influence over each other ; what 
takes place is effected by the vitreous and resinous electricities 
which cover them, and whose reciprocal action is confined to aug- 
menting or diminishing, upon certain parts of their surfaces, the 
pressure exerted there by the electricity against the surrounding 
air which retains it, or in general against the obstacles which oppose 
its change of place. After what is now laid down, if we continue 
to employ the words attraction and repulsion to denote the mo- 
tions of electrified bodies, the terms are to be considered as 
expressing simply the circumstances of these motions, and not 
as indicating the real cause on which they depend. 

The attraction and repulsion under consideration take place 
not only through the air ; they are exerted also through other 
non-conducting bodies, as glass and resin. If we suspend within 
a glass phial a stick of sealing wax rubbed and electrified, it at- 
tracts light bodies situated without the phial, ju?>t as if there 
were nothing interposed. This transmission manifests itself also 
through conducting bodies; but it is disguised under another 
phenomenon, of which we shall speak hereafter. 

To discover whether a given substance, on being rubbed in 
a certain manner, acquires the vitreous or resinous electricity, 
we must observe the effect it produces upon the electrical pendu- 
lum previously charged with a known electricity. For example, 
we touch this pendulum with a glass tube rubbed with woollen 
cloth ; and it receives the vitreous electricity. We rub with the 
same substance the body whose electricity is to be tried, and 
bring it toward the ball of the pendulum. If it repels the ball, 
its electricity is vitreous, if the ball is attracted, the electricity 
is resinous. We may vary the experiment if we choose, by 
first giving to the pendulous body the resinous electricity. 

As the signs of electricity in certain cases are very feeble, it be- 
comes desirable to increase the sensibility of the apparatus. This 
is effected by reducing the size of the pith ball, and suspending 
it by a fine silk thread. If we use, for example, one of the orig>- 
inal fibres, as they proceed from the silk worm, and not less than 
10 or 12 inches in length, a very weak electricity will be suffi- 
cient to put it in motion. There are still more sensible instru- 
E.&M. 2 



1 Electricity. 

ments, with which we shall become acquainted as we proceed, 
and by means of which we shall be able to comprehend the 
most delicate phenomena ; but the one above described will an- 
swer our purpose for the present. 

By subjecting to this proof a great variety of bodies, rubbed 
with different substances, we shall find that there is no constancy 
as to the kind of electricity developed, but that this depends as 
much on the nature of the rubbing substance, as on that of the 
body rubbed. Polished glass, for example, rubbed with woollen, 
acquires, as we have before said, the vitreous electricity ; but 
when rubbed with a cat skin, it takes the resinous electricity. 
Silk rubbed with resin, exhibits the resinous electricity ; rubbed 
with polished glass, it acquires the vitreous electricity. 

The several substances of the subjoined table take the vitre- 
ous electricity when rubbed respectively with the substance 
immediately following 5 and the resinous when rubbed with that 
which precedes: 



Cat skin, 


Paper, 


Polished glass, 


Silk, 


Woolen cloth, 


Gum lac, 


Feathers, 


Rough glass, 


Wood. 





It will hence be seen, that there is apparently no connection 
between the nature of the substance and the kind of electricity 
produced by it. 

The only general law which is known to exist among these 
phenomena, is, that the rubbing body and the body rubbed always 
take different electricities ; if the one be resinous, the other is vitreous, 
and vice versa. 

To ascertain this in any particular case, we must insulate 
the two bodies which are to be rubbed against each other. If 
they are solid, we fit to them handles of glass or resin, by which 
they may be held. It is well when it is possible, to give to the 
substances rubbed the form of plates, that the friction may take 
place over a greater surface. We may insulate and try in the same 
way a solid body and pieces of cloth, fur,&c, or two substances 
of the latter kind only, &x. When we have continued the fric- 
tion for a short time, we separate the two bodies ; and holding 



Two Kinds of Electricity. 1 i 

them always by the insulating handle, we present them success- 
ively to a very sensible electrical pendulum, charged with a 
known kind of electricity. We shall then find in every case 
that one of the substances attracts and the other repels the pen- 
dulous body ; the electricities are therefore different. Nume- 
rous experiments have been made to discover what are the cir- 
cumstances which determine a body to take the particular kind of 
electricity which it is found to possess, but without making known 
ing any thing very decisive. Sometimes the result is apparently 
determined by the most trifling circumstance ; when, for exam- 
ple, a piece of polished glass is rubbed against a piece of rough 
glass, the first takes the vitreous electricity, and the second the 
resinous, without any one being able to tell why the polishing of 
the surface should have this effect. If two ribands of white 
silk, taken from the same piece, are rubbed against each other 
crosswise, that which is rubbed transversely, acquires the resin- 
ous electricity, and that which is rubbed longitudinally, takes 
the vitreous electricity. Nothing further is known as to the ef- 
fect of the direction of friction. Indeed, the result is not always 
the same with the same bodies. iEpinus assures us that he has 
observed this fact in rubbing a plate of copper with one of sul- 
phur, and also in rubbing two squares of glass against each 
other ; when separated, they were always in contraiy states of 
electricity, but the same kind of electricity belonged sometimes 
to one and sometimes to the other. 

From these phenomena, we are led to the following curious 
experiment. Two persons are placed upon stools, called insu- 
lators, the feet of which are of solid glass or other insulating 
substance. One holds in his hand a dry cat skin, and with it 
rubs or strikes the clothes of the other, and thus acquires himself 
the vitreous electricity, while he gives to the other the resinous 
electricity, as may be proved by bringing near them succes- 
sively^ an electrical pendulum charged with a known kind 
of electricity. If a person not insulated touches the persons 
electrified, he will draw a spark from each of them. It is evi- 
dent that these phenomena can take place only while the electrL 
fied persons remain upon the insulating stool ; for if they leave 
it, they immediately impart their electricity to the earth. It is 
on this account, that when we insulate only one of the persons, 



1 2 Electricity. 

whether it be the one that rubs or the one that is rubbed, the 
insulated person only shows signs of electricity ; and if neither 
is insulated, no signs whatever are produced. It is manifest, 
moreover, that they ought to touch or communicate with each 
other only through the rubber. 

A cat skin is very convenient for this and many similar 
experiments, since it is very easily electrified. By passing the 
hand, in dry weather, over the back of a live cat, the hair stands 
erect, and is attracted to the hand ; sometimes, indeed, we even 
hear a crackling noise, and obtain small sparks. This takes 
place only in cold weather when the air is a good non-conductor. 
Dry hair is very easily electrified by friction, especially if it is 
fine and soft. 

Electricity is also produced by the friction of liquids against 
solids. To prove this, we place upon an air pump a cylindrical 
glass receiver, to the upper extremity of which is fitted a wooden 
cup, containing a small quantity of mercury. The receiver is then 
exhausted, and the mercury, being pressed by the external air, 
filters through the pores of the wood, and falls in a fine shower, 
which strikes against the sides of the glass cylinder. If we now 
present the electrical pendulum, suspended by its silk thread, we 
shall find that the part thus rubbed, is electrified. The cylinder 
should be perfectly dry, in order that it may retain all its elec- 
tricity, which is sufficiently feeble, when developed in this way, 
by the friction of falling mercury. 

We are hence able to account for an appearance often noticed 
in barometers which are well freed from air. Upon being in- 
clined in such a manner as to fill suddenly all the empty part of 
the tube, if the experiment is performed in the dark, a faint light 
is instantly seen, similar to that produced by a continued cur- 
rent of electricity through a vacuum. 

We may also obtain electricity by the friction of a gas against 
a solid body. If a current of atmospheric air be directed against 
a pane of glass, the glass takes the vitreous electricity. A dry 
silk handkerchief, on being shaken in the air, is electrified res- 
inously. 

Although friction is the most common, it is not the only means 
of developing electricity. It is produced also by a change of 
temperature, as in the fusion of metals and other substances. 



Laws of Electrical Attraction and Repulsion. 13 

Melted sulphur being poured into an insulated metallic vessel, is 
found, in cooling, to take the vitreous electricity, and the metal 
the resinous ; the phenomena are sometimes reversed, but the 
two kinds of electricity are always produced at the same time. 

Several crystallized minerals of a vitreous nature have also 
the property of becoming electrical when heated to a certain 
degree. One extremity of the crystal takes the vitreous elec- 
tricity, and the other the resinous, so that the parts where the 
two electricities prevail are separate ; still they are simultaneous- 
ly produced. 

Finally, electricity is also developed by various chemical com- 
binations, and indeed by the simple contact of all heterogeneous 
substances ; but this branch of the science requires much more 
complicated and delicate instruments than any of which we have 
yet spoken ; we shall therefore defer the consideration of it for 
the present. 



Of the Laws which govern the apparent Attraction and Repulsion of 
electrified Bodies. 

7. The phenomena of electrical attraction and repulsion 
being made known, the next thing to be done is to determine the 
laws according to which these forces are exerted at different 
distances. Here we have occasion to make use of the torsion 
balance, which has been successfully applied by Coulomb to 
the investigation of the laws of the variation of electrical and 
magnetic forces. 

The essential parts of this instrument consist of a vertical wire, 
the upper end of which is attached to a movable index, while 
the lower carries a horizontal needle. When very small forces 
are to be measured, they are made to act upon the extremity of 
this needle, and their intensity is estimated by the angle through 
which they cause it to move from its point of rest ; in other 
words, these forces are balanced by the force of torsion, which 
is always proportional to the angle of torsion.* 

* See note on the torsion balance. 



Fig. 5. 



14 Electricity. 

8. To apply this instrument to the measurement of electrical 
attraction and repulsion, we make the needle of gum lac, which 
is a very good non-conductor, and attach to one of its extremities 
a small ball b, of elder pith. Then, having placed the index to 
the graduated circle M against zero of its divisions, we turn 
the whole cap, together with the index, till the ball b is opposite 
to zero of the divisions traced upon the sides of the instrument.* 

This being done, we fix a second ball a at the extremity of 
a very small cylinder of gum lac, of such a length that being 
introduced vertically within the glass covering, this ball may 
reach the level of the other ; and it is to be so placed that 
the ball shall answer to zero of the lateral divisions, which re- 
quires the first ball to be moved from this point, one way or the 
other, through an arc equal to the sum of the radii of the two 
balls ; and the small torsion which results from this motion, is 
sufficient to keep them in contact. 

Now it is manifest, that if we touch these balls for an instant, 
or only one of them, with a body already electrified and insulat- 
ed, they will be electrified by communication, and both in the 
same manner ; they must therefore mutually repel each other ; 
but as the first only is moveable, the needle which carries it will 
turn through a certain arc, and after oscillating backward and 
forward a little, it will come to a state of equilibrium at a point, 
the distance of which may be read oft' upon the graduated 
paper. Thus the degree of torsion of the wire will counterbal- 
ance the repulsive force of the two bails, and will serve to meas- 
ure it. 



* These divisions are made upon a piece of paper, which is af- 
terwards pasted horizontally around the glass covering. If the cov- 
ering is circular, the divisions will be in degrees. But when we 
wish to introduce bodies of a considerable magnitude, glass cylin- 
ders, as they are commonly blown, are too small, and we make use 
of four vertical panes, which, together form a parallelopiped. In 
this case, the strip of paper containing the graduations, requires to 
be divided into tangents, zero being at that point on each pane, 
where the needle is perpendicular to the pane. 



Laws of Electrical Attraction and Repulsion. 1 o 

This is in fact the course to be pursued ; but as an extreme- 
ly small force is sufficient to twist a fine wire through a very 
great angle, it is obvious that the balls will require only a very 
small charge of electricity. For this purpose, we simply touch 
them with the head of a large pin, electrified by communication, 
the body of the pin being concealed in a stick of sealing wax ; 
the contact is effected by means of a small aperture in the glass 
covering made for this purpose, the stick of sealing wax serving 
as an insulating handle. 

Proceeding in this way, Coulomb found in one of his experi- 
ments that after the electricity was communicated, the needle 
described an angle of 36°. He then twisted the suspending wire 
in a contrary direction, so as to bring the needle to the distance 
of 1 8° from the fixed ball, and in order to this he was obliged to 
turn the index 126°. 

Finally, he twisted the wire so as to bring the needle to the 
angular distance of only 8|°, when the whole motion of the index 
was found to be 567°. 

During this experiment, the balls did not sustain any sensible 
loss of electricity. For by previous trials on the same day, 
Coulomb ascertained that electrified balls, diverging 30° from 
each other, lost only one degree of their divergence in thi*ee min- 
utes ; and as he employed only two minutes in making the above 
experiment, we may safely neglect as insensible the diminution 
of electricity sustained by the balls, either on account of the 
contact of the air, or by loss along the supports. This was ow- 
ing, as we shall see by and by, to the dryness of the air at the 
time of the experiment, and to the excellent choice of the insu- 
lating supports. 

In order to obtain the results to be derived from this experi-Fig. 6 
ment, let us represent by ah d the circumference described by 
the moveable ball b ; let c be the centre of this circumference, 
and let us take the arc a b equal to 36°, the first distance to 
which the ball was repelled. It appears that in this case the 
repulsive force of the two balls, was counterbalanced by a tor- 
sion of 36°, exerted in the direction ab ; for by the arrangement 
made at the commencement of the experiment, the torsion is 
nothing when the needle is directed toward the point a. 



16 



Electricity. 



In the second case, the wire was twisted 126° in the direction 
b a. If the needle were free, this torsion would carry it to d', 
1 26° beyond the point a ; but, on the contrary, the repulsive 
force retains it at b' 18° this side of a. Therefore at this point 
the repulsive force of the two balls would hold in equilibrium a 
torsion of 126° + 18° or 144°. 

Finally, in the third case the torsion indicated by the graduated 
circle, was 567°, always in the direction ba; but instead of go- 
ing 567° beyond the point a, the needle stood at 8|° on this 
side of the point ; thus the repulsive force which kept it at that 
distance w r as equivalent to a torsion of 

567° -f 8 J° or blb\°. 

Accordingly we have in the following table the relative tor- 
sions and distances. 



Angular distance of the 
two balls. 


Measure of the repul- 
sive force by the tor- 
sion. 


" ' 

36° 
18° 
8i° 


36° 

144° 

5751° 



A remarkable law is hence manifest. The angular distances, 
contained in the first column, are nearly as the numbers 1, |, j, 
while the corresponding torsions, which measure the effect of the 
repulsive forces upon the needle, are as the numbers 1, 4, 16, 
that is, inversely proportional to the squares of the preceding. 
These ratios, therefore, make it evident that the electrical forces, 
like the attractions of the heavenly bodies, are in the inverse ra- 
tio of the squares of the distances. 

Strictly speaking, the distance of the two balls is the chord 
of the .arc by which they are separated, and not the arc itself. 
Moreover, the repulsive force which they exert upon each other 
acts obliquely, and consequently is not wholly employed in pro- 
ducing the divergence. But this obliquity is very small in our 
experiments, on account of the small extent of the arcs ; and for 
the same reason, there is very little difference between the arcs 
and their chords. It will hence be perceived, that our conclu- 



Laws of Apparent Attraction and Repulsion. 1 7 

dons are fairly made out, But we may put the subject beyond 
all doubt, by performing the calculation in a rigorous manner. 
We thus find, that where the arcs of divergence do not exceed 
36°, the ratios deduced from the arcs, and those obtained from 
the rectilineal distances, do not differ by any sensible quantities. 
Confining ourselves, therefore, within these limits, we may apply 
the law of the squares of the distances to the arcs themselves, 
and thus very much simplify the calculations. 

9. The wire employed by Coulomb in his experiments was 
of silver, and on account of its fineness, its sensibility as to tor- 
sion was very great. Other instruments still more sensible were 
invented by the same philosopher for the purpose of indicating 
the minutest quantity of electricity. These instruments, which _. 7 
we shall call electroscopes, are true electric balances, in which a 
single «fibre of silk, as it comes from the silk-worm, takes the 
place of the metallic wire, while the needle is a small thread of 
gum lac, about an inch in length, terminated at one of its ex- 
tremities by a very small disc of tinsel.* In one of these 
instruments used by Coulomb, the weight of the needle and 
the tinsel together did not exceed \ of a grain. A fibre of silk of 
four inches in length, has such a flexibility, that with a lever 
an inch long, it requires a force equal only to the sixty-thou- 
sandth part of a grain to twist it 360°. To communicate the elec- 
tricity to the disc, we pass through a stick of sealing wax, a cop- 
per wire, terminated at one end by a small ball of elder pith gilt, 
and at the other by a metallic ball, or by a hook the point of 
which returns into the wax. This stick thus armed is introduced 
into the glass covering, the hook being outward, and it is so fixed 
that the centre of the gilt ball, being seen in the direction of the 



* These threads are easily formed, by warming in the flame of 
a candle the middle of a small stick of gum lac, and holding it at the 
same time by its two extremities. When the resin begins to melt, we 
pull the two ends rapidly asunder, and the melted matter is common- 
ly drawn out into a very fine thread, which adheres to the two sol- 
id ends. In the same way we draw out threads of sealing wax and 
even of glass ; but for the latter substance, unless we employ a tube 
already very fine, the heat of a candle is not sufficient, and we are 
obliged to use a blow-pipe. 
E. & M. 3 



18 Electricity, 

suspending wire, answers to zero on the sides of the covering. 
When the needle is at rest, we turn gently the index to the grad- 
uated circle till the tinsel conies in contact with the gilt ball; 
the instrument is then ready for use. If we communicate elec- 
tricity to the hook, it is transmitted to the ball and to the tinsel 
disc, which is immediately repelled. The sensibility of these 
electroscopes is such, that if after having electrified by friction 
a stick of sealing wax, it is presented to the exterior hook, even 
at the distance, for example, of three feet, the needle is imme- 
diately repelled more than 90°. We shall see hereafter how 
electricity may be thus developed at a distance without any con- 
tact. At present we give this result only as a proof of the ex- 
treme sensibility of the instrument. By means of this electro- 
scope, it would be easy to repeat all the experiments mentioned 
in the preceding chapter, on the nature of the electricity excited 
in different bodies by their mutual friction. 

10. After having determined the laws of electric repulsion, 
we naturally direct our attention to those of attraction exerted 
between bodies charged with different electricities ; and here 
also we follow the example of Coulomb. But in this case the 
balls must not touch each other in their first position before be- 
ing electrified ; on the contrary, they must be separated, and 
th^ torsion must prevent them from uniting. For this purpose, 
Fig. 8. we begin with taking away the fixed ball a ; and by means of 
an insulated pin-head, we give to the moveable ball an elec- 
tricity of a certain kind, for example, the resinous. This being 
done, we turn the index through a certain known angle c; 
the wire being free will follow this motion, and after some 
oscillations, the extremity of the needle will come to a state of rest 
before a certain point b of the lateral divisions, which will be c 
degrees distant from its first position. This operation will there- 
fore have transferred the zero of torsion through the known an- 
gle c in the direction a b. 

We now replace the fixed ball a and give it a different elec- 
tricity from the former, that is, the vitreous. The two balls 
being attracted toward each other, the needle will move toward 
the fixed ball a, and if an equilibrium be possible, it will stop at some 
point b'. We note this point, and then turn the graduated cir- 
cle backward and forward through known angles, for the purpose 



Lazvs of Apparent Attraction and Repulsion, 1 9 

of varying the torsion, and we note in each case the points where 
the needle becomes stationary. Comparing these torsions and 
distances, as in the experiment on the variation of repulsive forc- 
es, we shall find that the same law obtains in both. We con- 
clude, therefore, that the attractive forces exerted between dif- 
ferent electricities, like those of repulsion in the case of electric- 
ities of the same kind, are inversely as the squares of the dis- 
tances. 

In the above experiment, a precaution is to be observed, 
without which we should not succeed. When the attractive 
force of the two balls causes them to approach each other, the 
intensity of the attraction increases as the distance becomes less, 
and if no other cause came into operation, they would come in 
contact. But the torsion is opposed to their approach, and the 
resistance increases as the needle departs from the point b to- 
wards the other ball. Now within a certain distance, this resis- 
tance does not increase rapidly enough to overcome the increase 
of the force of attraction, so that an equilibrium being impossible, 
the balls having reached this point, approach more and more, 
and finally unite. A very simple calculation would make this 
evident, and determine the limits of departure to be observed. 

It even happens, sometimes, that they still unite under the cir- 
cumstances in which, according to the calculation, an equilibrium 
is possible. This takes place because the suspending wire admits 
of an oscillation in the needle, for some time, about the point of 
equilibrium where it must finally stop. If the extent of these 
oscillations be such as to carry the moveable ball sufficiently 
near to the fixed ball for the attraction to increase more rapidly 
than the force of torsion, this torsion will not be sufficient to bring 
back the needle, and the balls will come in contact. 

11. Coulomb has also determined the law of electric at- 
traction by another method, which I shall describe here, be- 
cause it serves to verify the preceding, and also because it will 
be of use when we come to treat of magnetism. It consists in 
suspending horizonally, by a single fibre of silk, a needle of gum 
lac, the extremity of which carries a disc of tinsel, which is to 
be electrified. Before this needle, at some distance, we place a 
globe charged with a different electricity, which attracts it and 
causes it to oscillate in virtue of its action. We then determine 



20 Electricity. 

by calculation the attractive force at different distances of the 
electrified globe, from the number of oscillations of the needle 
which take place in a given time, just as we determine the force 
of terrestrial gravity from the number of oscillations of the 
MacL. common pendulum. The results thus obtained confirm the law of 
the inverse duplicate ratio of the distance, before discovered by 
means of the torsion balance. 

1 2. The same method would serve also to determine the law 
of repulsive forces ; for by communicating to the globe and to 
the disc similar electricities, the disc will be repelled, the direc- 
tion of the needle will be inverted, and it will oscillate in virtue 
of this repulsion in a direction diametrically opposite to the for- 
mer ; but with the exception of this turning, which will affect 
the distance of the disc from the globe, the observations and the 
calculations will be the same as in the preceding case. 

By means of the results which we have obtained, we can 
calculate, for all possible distances, the force of attraction or re- 
pulsion of two electrified balls, when we have determined this 
force for a single known distance. 

But this gives us only the measure of the total effect ; we do 
not know what proportion each ball contributes. Still, unless 
they are perfectly equal and equally electrified, it is manifest 
that they must contribute unequally- It is proposed, therefore, 
to discover this proportion ; which is readily done, if we could 
give to one of the balls, or take from it, a portion of electricity 
having a known ratio to that which it had before. For, by 
measuring the new torsion which produces an equilibrium in this 
new state, and comparing it with that which took place before 
at the same distance, we should discover what influence the prop- 
er electricity of each ball has upon the total effect. Now it is 
very easy to take from each ball half its electricity. To this 
end, we have only to touch it for an instant with another ball 
of the same substance, of the same diameter, and equally insu- 
lated ; for it is manifest that the two balls being perfectly similar, 
the electricity will be equally divided between them, so that after 
the contact, the proper action o^the ball touched will be less by 
one half. Now by proceeding in this way, we find that the total 
force of attraction or repulsion, which was at first exerted be- 
tween this ball and the fixed ball of the balance, is, after the 
contact, reduced exactly one half. 



Laws of Apparent Attraction and Repulsion. 21 

This method of reduction is not confined to balls, but ex- 
tends to circles, and probably to all bodies whose figure or dis- 
tance asunder is such, as to admit of their being considered as 
points. Coulomb substituted instead of the fixed ball of the bal- 
ance, an iron circle £ of an inch in diameter, leaving always a 
pith ball at the extremity of the needle. He electrified these 
two bodies simultaneously by means of the head of a pin, and 
the repulsive force separated the needle from the circle ; when 
it was brought back and placed at a distance of 30°, the index 
pointed to 310°; the repulsive force therefore was 140°. He 
then touched the little iron circle by another of the same sub- 
stance and same diameter ; the needle immediately approached 
the circle, and to bring it back to the distance of 30°, it was 
found necessary to untwist the wire till the index stood at 40°; 
therefore the repulsive force was reduced to 40° -f- 30° or 70°, 
the half of 140°, the measure of its former intensity. 

13. By these experiments, moreover, we are made acquaint- 
ed with a remarkable fact, namely, that the distribution takes 
place in exactly the same proportion, whatever be the substance 
of the conducting bodies placed in contact, provided their forms 
and dimensions are the same. Coulomb touched the fixed pith 
ball with a ball of the same size of copper and of several other 
substances ; he touched the iron circle also with a circle of paper 
of the same diameter; and the distribution was always into equal 
parts. 

14. These experiments lead to two important conclusions. 
The first is, that the total force of attraction or repulsion, vary- 
ing for each distance in the same ratio as the quantities of elec- 
tricity belonging to the two bodies, it follows necessarily that the 
expression for the force in question is proportional to the pro- 
duct of these two quantities. Then each ball or each circle 
contributes to the entire force which attracts or separates them, 
according to the value of the factor which it introduces 
into this product. In future we shall call this factor the 
electrical reaction of the ball or circle of which it measures the 
action, and we shall extend by analogy the same denomination 
to all bodies of whatever form, when we observe their elec- 
trical action at so great a distance that they may be considered 
as simple points. 



• 



22 Electricity. 

15. The second conclusion is, that, since the distribution of 
electricity between conducting bodies of the same size and fig- 
ure, takes place always in equal proportions, whatever be the 
nature of the substance, it follows that these bodies do not act 
upon electricity by a chemical affinity depending on the nature 
and arrangement of their material particles, but are, with respect 
to it, merely vessels or receptacles in which it distributes itself 
mechanically according to its own proper laws. 



Of the Laws according to which Electricity is dissipated by the Contact 
of the Air, and along the Supports which retain it but imperfectly. 

16. The general law of electric attraction and repulsion will 
be understood from what precedes ; but to verify with exactness 
the consequences to be deduced from it, and to follow out the 
electric principle into its most minute effects, we must assure 
ourselves of its uniform intensity, or at least determine the laws 
according to which this intensity diminishes by contact with 
the air and by the imperfection of the insulating supports. 
Such is the object of this section, for the substance of which 
we are still indebted to the labours of Coulomb. 

1 7. When an electrified conducting body rests on insulating 
supports, its electricity diminishes more or less rapidly, and 
finally disappears. Many causes conspire to produce this effect. 
In the first place, there probably dees not exist in nature a per- 
fectly insulating substance ; for we know no one which does not 
transmit, at least along its surface, a strong electricity ; glass, 
sealing wax, even gum lac in this way transmit it sensibly, al- 
though with difficulty. Of this we may satisfy ourselves by 
forming cylinders of these substances ; and holding them suc- 
cessively for some time in contact, at one extremity, with the 
prime conductor of an electrical machine. For, upon taking away 
the cylinder and presenting this same extremity to the needle 
of the electroscope, we shall see that it is impregnated with the 
electricity of the conductor ; and even on cutting off the end of the 
cylinder, we shall still find that the electricity is also propagated 



Dissipation of Electricity, 23 

over the rest of the surface to a certain distance, with decreasing 
intensity. 

All the supports employed to insulate an electrified body, 
must draw off more or less of the electricity ; and if they are 
short enough to be thus electrified throughout their whole length, 
they will cause a gradual but continual waste, so that from this 
circumstance alone the electrical reaction of the insulated body 
must become weaker and weaker. 

18. Secondly, electrified bodies are always surrounded and 
in contact at every point of their surfaces, with the atmosphere 
which transmits the electricity with greater facility, according 
to the quantity of aqueous vapour which it contains ; and per- 
haps, according to modifications arising from heat and other cir- 
cumstances, in the very properties of its chemical elements, so 
that we may generally regard it as composed of an infinity of 
more or less perfectly conducting atoms. Accordingly, each 
particle of air which touches an electrified body must take a 
part of its electricty. But after it is impregnated in the propor- 
tion which belongs to its magnitude and conducting power, it is 
immediately repelled, and its place is taken by another, which 
is also electrified and driven away in its turn ; and thus by the 
effect merely of these successive contacts, continually renewed, 
the electricity of bodies must diminish, according to a progres- 
sion depending on the conducting power of the air. 

Finally, the aqueous vapour suspended in the atmosphere 
contributes to this dissipation in another way ; for it attaches 
itself to the supports in greater or less quantity, according as it 
is more or less abundant, and according as the matter of the 
supports has greater or less affinity for water. Such of these 
particles as are nearest to the electrified body, receive the elec- 
tricity from it immediately ; and if the force with which thej r 
are then repelled by it is less than their adhesion to the sup- 
port, they must transmit this electricity in part to the neigh- 
bouring particles, and they, in the same w T ay to the next ; so 
that all these particles being good conductors, form, as it were, 
a chain upon which the electricity must go on decreasing from 
the conducting body, but which, nevertheless, will finally con- 
duct it to the ground, if the support is not long enough to prevent 
it. If the particles which, form this chain are nearer to each other 



24 Electricity. 

than those in the air itself, which is often the case, the electric- 
ity will be dissipated more rapidly along the support than by 
the contact of the air ; and this frequently happens, as we shall 
presently see. 

19. Whatever difficulty there may seem to be in guarding 
against the last cause of dissipation, it will be seen to be indispen- 
sably necessary to do it in order to be able to determine the loss 
of electricity occasioned by the contact of the air simply, and for 
the purpose also of making allowance for this same cause of 
waste, in experiments where it is blended with the loss occasion- 
ed by the supports. The only means of effecting so important 
an object, is to choose for supports the substances which insulate 
best, and to make them so small as to admit, in contact with their 
surfaces, but few particles of water or other conducting matter 
in comparison with the surrounding atmosphere ; for in this case 
the support will insulate, to say the least, as well as the air, and 
the extent of its contact with the electrified body being very 
small we may neglect its effect entirely. 

By several experiments conducted upon this principle, Cou- 
lomb found, that when the intensity of the electricity was not 
very great, a small cylinder of sealing wax or of gum lac, ^\ of 
an inch in diameter, and 1^ inch in length, was almost always suf- 
ficient to insulate perfectly a pith ball of | of an inch in diameter. 
For the electricity was not dissipated any faster when the ball was 
supported by several of these cylinders, than when it was sup- 
ported by one only, although the facility for dispersion was in- 
creased with the number of points of contact. He ascertained 
also that when the air was dry, a very fine thread of silk drawn 
through boiling sealing wax, and thus forming a little cylinder of 
not more than ¥ * ¥ of an inch in diameter, answered the same pur- 
pose, provided its length was 5 or 6 inches. A thread of glass 
drawn out in an enameller's lamp to 5 or 6 inches in length, will 
not insulate the ball except in very dry weather, and when it is 
feebly charged with electricity ; the same may be said with res- 
pect to a hair or a silk thread, at least if they are not covered 
with sealing wax, or, which is better, with pure gum lac. 

20. After making these preliminary experiments, Coulomb 
soldered the fixed ball of the balance to the end of a thread of 
pure gum lac 1 I inch in length, which was terminated with a 



Dissipation of Electricity. 25 

very fine thread of silk covered with sealing wax, so that this 
ball might be considered as perfectly insulated. The moveable 
ball was no less so, since the needle which carried it was also a 
very fine cylinder of gum lac. Coulomb first made these two 
balls of equal diameter, and he employed a balance of such sen- 
sibility, that the torsion of an entire circumference, answered to 
a force at the extremity of the needle of 3 £ ¥ of a grain. The 
zero of torsion of the wire being brought to the centre of the 
fixed ball, and the two balls being in contact, they were touched, 
as in the former experiment, with the electrified head of a pin ; 
the moveable ball was repelled, and, after several oscillations, 
fixed itself at a certain distance from its point of departure, for 
instance, at 40°. 

The suspending wire was then twisted so as to bring it 
back to a less distance, as 20°, for example. To do this, it was 
necessary to turn the index of the graduated circle 140°. Thus 
the torsion, equal to the repulsion of the two balls, was 1 40° -f- 
20° or 160°. 

The moment when the moveable ball stopped at this distance, 
was observed with a seconds-watch, and found to be 50 minutes 
after 6. 

As the electricity is dissipated by the contact of the air, the 
repulsive force of the balls gradually diminishes ; and after some 
minutes they become nearer to each other than 20°. To bring 
them again to this distance, we untwist the wire by a known 
quantity, for example, 30°. Its force of torsion being diminished 
by this quantity, the moveable ball is driven further off than 20°. 

We wait till the loss of electricity brings it back to this 
distance, and observe the time. This was found to take place 
at 53 minutes after 6, and consequently, 3 minutes after the 
first observation ; the force of torsion then equal to the repulsion 
of the two balls, becomes 

140°— 30° + 20° or 130°. 

The loss of repulsive force between the two experiments, was 
therefore equal to 160° — 130° or 30°, that is, to the quantity 
by which the wire was untwisted to bring the balls to the same 
distance. This effect was produced in 3 minutes ; and as in 
small intervals we find it is proportional to the times, it follows 
E. &M. 4 



26 Electricity. 

that the loss is 10° a minute. Moreover the mean repulsive 

force between the two experiments, was ~ or 145°. 

Comparing this with the observed diminution, we see that the 
electrical force of the two balls diminished on this day by 

— - or - — a minute, on account of the contact of the air only. 
145 14^ 

By experiments of this kind, Coulomb constantly found that 

on the same day, and with the same state of the air, the loss of 

electricity for a short time was proportional to its intensity, and 

that thus the ratio of these two elements is invariable. But this 

ratio changes with the state of the hygrometer, and consequently 

with the quantity of aqueous vapour suspended in the air. 

21. A greater number of experiments on this subject would 
serve to discover the ratio between the quantity of aqueous va- 
pour, and the greater or less rapidity with which the dispersion 
of electricty takes place. We might thus determine also whether 
this vapour is the sole cause of the phenomenon, or whether the 
pressure and temperature of the particles of the air itself are not 
also concerned. If we were able to estimate the influence of these 
different causes, we should perhaps find the electrical balance to 
be the most exact and sensible of all hygrometers. We might 
at least, from the simple indications of meteorological instru- 
ments, assign the proportional loss of electricity sustained. For 
want of these data we are obliged to determine this proportion 
directly by experiment for each particular time, when we have 
occasion for exact experiments on the intensity of electrical 
forces. 

22. It is very fortunate for us in our experiments, that the 
law of decrease happens to be so simple ; since, for the same state 
of the air, it is proportional to the repulsive force, we have oc- 
casion only for a single experiment each time, in order to ap- 
ply the necessary correction to any number of cases. Moreov- 
er, the law which we have discovered, enables us, when the in- 
tensity of an electrical force and its rate of decrease are once 
determined, to calculate it for any other given moment. By ex- 
amining the results thus obtained, we learn that the same law 
of decrease is applicable to cases where the two bodies acting 
upon each other, are of unequal magnitudes, and charged with 



Dissipation of Electricity. 27 

unequal quantities of electricity. Indeed, whatever be the mag- 
nitude of the fixed ball compared with the moveable one, and 
whatever be the quantity of electricity at first given to them, 
whether they are electrified simultaneously, or one after the 
other, and in whatever proportion, the momentary decrease of 
their whole repulsive force, measured at the same distance, is al- 
ways in the same proportion to its intensity ; and thus our ex- 
periments are all equally suited to the purpose of finding this 
common ratio. Moreover, this ratio is still the same when we 
employ balls of different substances. The nature of the substance 
has absolutely no influence on the loss of electricity occasioned 
by the contact of the air, at least with respect to the portion 
which acts at a distance by attraction and repulsion ; and this 
confirms the observation which we have before made, that mate- 
rial bodies do not seem to retain the electric principle by any 
proper affinity, but by the effect simply of the resistance which 
is opposed to it by the surrounding air. For example, in weath- 
er when the electricity was decreasing at the rate of T \ a minute 
for each of the pith balls of the balance, Coulomb found that it 
decreased also j\ when he substituted for one of these balls a 
ball of copper ; and, which will appear still more extraordinary, 
the decrease was also T \ for a ball of sealing wax, which had 
been charged with electricity, by bringing it in contact with a 
body strongly electrified ; and thus the surface of such a body 
opposes no difficulty, to the transmission of the electric principle, 
and has no influence in retaining the portion of this principle, 
which manifests itself by its reaction, when once it becomes 
free. 

23. We have as yet considered only bodies of a globular 
shape ; but whatever may be the figure of the electrified body, 
whatever its magnitude and the distribution of its repulsive force, 
if the air is very dry and the electricity communicated not very 
intense, the momentary decrease of the repulsive force is always 
the same, and preserves always the same ratio to its intensity. 
This was demonstrated by Coulomb with a globe of a foot in 
diameter, and with cylinders of all diameters and all lengths. 
He substituted for the balls of his balance, circles of paper or 
metal ; he also, in one instance, armed one of them with a cop- 
per wire f of an inch in length, and ^\ in diameter ; and he 



28 Electricity. 

found that, at the time of his experiments, the repulsive force of 
all these bodies, although so different in form, decreased by the 
same quantity, namely, T i F in a minute. But it is necessary to 
remark, that this equality of decrease for bodies of different 
forms takes place only when their electricity is already consider- 
ably reduced, and reduced so much the more, according as the 
air is more moist. For all angular bodies, when possessed of a 
strong electricitjr, lose at first this excess by a much more rapid 
decrease, as we shall have occasion to show hereafter, when we 
come to speak of the electricity of points. This phenomenon 
may be rendered evident to the senses, without the aid of the 
balance, by connecting the prime conductor of an electrical ma- 
chine with a metallic bar, having sharp angles or points. For, 
upon putting the machine in motion, the experiment being per- 
formed in the dark, the electricity communicated to this bar, 
will produce, as it flies off from the points, beautiful tufts of light. 
I do not mean to say that this fire is itself the electricity, for 
herein is involved a question to be examined hereafter ; but as 
light always attends the rapid escape of electricity, it is at least 
a sign and indication of this escape. It would be well worth our 
attention to inquire whether, the state of the air being the same, 
the two kinds of electricity are dissipated at the same rate. I 
have made the examination, and find that this is in fact the 
case. 

24. The law of the gradual dispersion of electricity, produc- 
ed by the mere contact of the air, being thus known, Coulomb 
proceeded, according to the same method, to determine that oc- 
casioned by the imperfect insulation of the supports. 

The course which first suggests itself, is to choose such sub- 
stances for supports, that the loss arising from this cause shall 
be very great, compared with that depending upon the contact 
of the nir. But this very rapid decrease w r ould be attended with 
a serious inconvenience. For every time we touch the balance, 
either to give the balls their first electricity, or to change the 
torsion by means of the graduated circle, the needle does not re- 
turn to a quiet position till after several oscillations. It is therefore 
necessary that the insulation should be pretty perfect, that the 
electricity may not sustain in this interval very great variations 
of intensity, and that we may be able to make several experi- 



Dissipation of Electricity. 29 

inents of this kind successively, without giving to the balls a 
new charge. Accordingly, Coulomb instead of suspending the 
fixed ball of the balance to a cylinder of gum lac, attached it to 
a single fibre of silk, as it comes from the silk worm, of about 
fifteen inches in length. The moveable ball at the end of the 
needle was always insulated as perfectly as possible, and made 
equal in magnitude to the other. Coulomb measured, as before, 
the repulsive force of the two balls at different times, and hence 
calculated the decrease of the electricity. He found this de- 
crease to be much more rapid than that produced by the air 
alone, when the intensity of the repulsive force was considerable, 
but that it became gradually less rapid as the intensity dimin- 
ished ; and thus at a certain point, the ball, supported by the 
silk fibre, lost precisely as much as when it was insulated in the 
most perfect manner ; and this limit being once attained, the 
same equality continued through the lowest degrees of intensity. 
We hence learn, that at this point the thread begins to insulate 
perfectly. 

In these experiments, the moveable ball can lose its electric- 
ity only by the contact of the air. We may therefore calculate 
for any instant, the state of its electrical action from the law of 
decrease above established ; and as the whole repulsive force, 
obtained by observation, for this instant, makes known the amount 
of the reciprocal electrical action of the two balls, we can thence 
deduce, for the same instant, the electric action of the fixed ball. 
By this calculation, therefore, the effect of imperfect insulation 
is determined. Applying it to the experiments we have mention- 
ed, Coulomb was able to fix the degree of electrical action at 
which each of the supports used by him began to insulate per- 
fectly; and he found that the intensity of this action was pro- 
portional to the square root of their respective lengths ; in other 
words, that for the same state of the air, a quadruple length of 
support insulates perfectly a double quantity of electricity ; it 
being well understood that this proportion is restricted to sup- 
ports of a cylindrical form, which differ only in respect to length. 
When the substance or its figure is changed, it is necessary to 
deduce the ratio from the formula itself. Calculating in this 
way, from experiment, the intensity of the electrical action, at 
which perfect insulation begins, in the case of threads of gum 



30 Electricity. 

lac and of silk of the same length and diameter, we find that it 
is ten times greater for the first substance than for the second. 
By similar calculations we may compare together the conduct- 
ing power of all substances which transmit electricity imper- 
fectly. 

In order thus to compare one substance with another, it is 
by no means necessary that the balls of the balance should be 
observed at the same distance in the two series of experiments ; 
it is enough that this distance be constant in each series, and 
that we substitute its value each time in the formula. It is 
equally immaterial what degree of electricity we give to the 
balls. But it is always necessary that they should be equal and 
simultaneously electrified ; it is also necessary that they, as well 
as the torsion wire, should be the same in all the experiments ; 
otherwise the ratio of the torsions to the repulsive forces would 
not be the same in the different series, which would render the 
comparison of them more difficult and less direct. These are 
the only indispensable precautions to be observed. 



Of Electricity in a State of Equilibrium in insulated Conducting 

Bodies, 

25. Knowing how to reduce the electrical action of bodies to 
a constant state, notwithstanding the continual loss which takes 
place by the contact of the air, and along the supports, we are 
prepared to inquire into the mode in which electricity distrib- 
utes itself among the different parts of the same body, both in 
its interior and at the surface. 

Now, from what we have already learned upon this subject, 
it would seem very probable that the electricity is confined en- 
tirely to the surface of conducting bodies, and that their interior 
particles have no effect in retaining it ; otherwise, it is not easy 
to perceive how the mere circumstance of equality of surface in 
the case of two bodies in contact, should produce between them 
an equal division of electricity, whatever be the substance of 
the bodies themselves, or how this equality should take place 
when one of the bodies is solid and compact, and the other hoi- 



Electrical Equilibrium. 31 

low and presenting scarcely any thing but a simple surface ; 
whereas all these things become simple and intelligible, on the 
supposition that the electricity, in a state of equilibrium, is dif- 
fused only over the surface of bodies, without penetrating into 
the interior. 

26, This property, suggested to us by analogy, is of sufficient 
importance to be made the subject of direct investigation. 

It may be rendered evident, in the first place, by the follow- Fig. 9. 
ing experiment. Take a conducting body S of a spheroidal fig- 
ure ; form in like manner, of a conducting substance, two very thin 
caps jE, jE, of gilt paper, for instance, and give them such a shape, 
that being joined, they will exactly cover the body S, attaching to 
them tubes of gum lac EM, EM, by which they may be removed 
and replaced, without being deprived of their electricity. This 
done, put the body S upon an insulating support, or suspend it by 
a very fine silk thread covered with gum lac, and give it any 
portion whatever of electricity. Then, after touching tke two 
caps to make it certain that they are not electrified, place them 
upon the spheroid £, holding them by the extremities of their 
insulating handles ; after a moment's contact, withdraw them, and 
present them to an electrical pendulum. We shall find that they 
have taken off the electricity of the spheroid, and taken it en- 
tirely. 

27. We may also verify this property in another and more gene- 
ral way; for the body, submitted to trial, may have any form what- 
ever, and the experiment be made without taking from it any of 
its electricity. We have only to pierce the surface of this body 
with one or more small cylindrical holes £ of an inch in diameter 
and of any depth ; we next draw out a thread of gum lac of several 
inches in length, to the end of which we attach a small circle of 
gilt paper, like that of the needle of the electroscope, and having 
a diameter of i or I of the size of the holes. This being done, 
we insulate the body S, and electrify it strongly by sparks from 
the prime conductor of an electrical machine, or in any other 
way ; then holding the thread of gum lac by its free extremity, we 
carefully introduce the gilt circle attached to it into one of the 
openings of the body S, taking care not to touch the edges of the 
opening. Upon withdrawing the circle, it will be found not to 
possess the minutest portion of electricity. But if. after having 



32 Electricity. 

repeated this experiment with the same result, we touch the cir- 
cle for an instant to the exterior surface of the body 5, or only 
to the edge of one of the cavities, it will be seen to exert a lively 
action upon the needle of the electroscope. We infer, therefore, 
that the electricity of the body S resides wholly on its surface, 
and not at all in the interior. Not only is there none in the in- 
terior, but it is impossible to fix any there ; for if we charge 
directly the circle of gilded paper, by taking the electricity 
from another body or from the exterior surface of the body S, 
and then introduce it into the cavity of this body, all the elec- 
tricity which it had acquired abandons it, and passes into the 
body enveloping £, where it immediately gains the exterior 
surface ; and the little plane being withdrawn from the cavity 
where it was introduced, is found to be discharged. 

This result applies generally to all bodies of whatever figure, 
but on repeating the experiment, we sometimes find that the gilt 
circle, on being withdrawn from one of the cavities, shows some 
feeble signs of an electricity of a contrary nature to that of the 
body S, and which does not disappear even when we touch the 
circle in order to discharge it. The circumstance of this elec- 
tricity being thus permanent, proves that it does not belong to the 
gilt circle itself, but is communicated to it by the gum lac, which 
restores it as fast as it is taken off; and accordingly we can de- 
rive from it no evidence of the existence of electricity in the 
interior of the body S. Now how could the thread of gum lac, 
which carries the circle, without touching the edges of the aper- 
ture and by proximity alone, thus acquire an electricity con- 
trary to that of the body S ? This phenomenon will be explain- 
ed soon, when we come to treat of the development oi electricity 
at a distance. For the present I shall merely observe, that this 
effect, which is purely acccidental, is almost always insensible 
when the gum lac is pure, the air dry, and the cylinder suffered 
to remain in the cavity only for a short time. 

We may rest assured, therefore, that the electric principle, 
whatever it may be, resides on the surface of conducting bodies, 
and not in their interior. We know, moreover, by further ex- 
periments, that the air retains it upon the surface, and is the 
only obstacle which prevents its escape from the body. Hence, 
combining these two facts, it will be seen that the electric princi- 



Electrical Equilibrium, 3,3 

pie always distributes itself over conducting bodies in a very 
thin stratum, whose exterior surface, being contiguous to the air, 
and confined by the pressure of this fluid, is the same as that of 
the electrified body, while the inner surface, almost coincident 
with the other, since the stratum is very thin, must be determin- 
ed according to other laws, to be deduced from observation. 

28. For example, when the electrified body is a sphere, the 
circumstance of symmetry alone requires that the inner surface 
should also be spherical and concentric with the outer ; for it must, 
like the body itself, be symmetrical in every direction about the 
centre. When we accumulate successively upon a sphere great- 
er and greater quantities of electricity, we may suppose, either 
that the newly added quantities dispose themselves spherically 
under the first, and augment the thickness of the stratum, or that 
the thickness remaining the same, the density of the electricity 
is augmented at each point. It is of no importance, in practice, 
which way we consider it ; for the thickness of the stratum being 
always very small, all the electrical particles collected under 
each infinitely little superficial element, must act by attraction 
or repulsion upon external bodies just as if they were all con- 
centrated in a single point, and consequently as if they were in- 
finitely condensed. Thus their action will always be proportion- 
al to their number, in whatever way they are regarded. But, 
considering the subject physically, the notion of a thickness es- 
sentially limited does not seem natural ; for there is no obstacle 
in the interior of a conducting body, to prevent the electricity 
from spreading in that direction ; if it does not so spread itself, 
it must be on account of the laws of its equilibrium; and for this 
reason it becomes extremely probable that for each given quan- 
tity of electricity, the thickness of the electrical stratum depends, 
in like manner, upon these laws. 

29. The above method of trying the electricity of a conduct- 
ing body, by touching it with a circle of gilt paper, insulated at 
the end of a thread of gum lac, is applicable to a variety of cases. 
It will serve to show, not only the existence and the nature of this 
electricity, but also the absolute quantity which may be collected 
upon each point of the surface. For this purpose, instead of 
presenting the little plane to the electroscope, as in the preced- 
ing experiment, we substitute it for the fixtxl ball of the balance 

E. 8/ M. 5 



34 Electricity. 

and observe its action upon the moveable ball or circle, pre- 
viously charged with electricity of the same kind. The small 
magnitude of these bodies, permitting us to consider them as points, 
it is manifest that the electrical action of the small plane will be 
proportional to the quantity of electricity with which it is covered ; 
and if we always introduce it into the balance without any loss 
being sustained by the moveable ball or circle, the torsions nec- 
essary to bring them successively to the same distance will give 
the ratios of the different charges. Now, since a very small 
plane applied to a body is confounded with an element of its 
surface ; we must presume that these charges will also be pro- 
portional to the charges of the points where the circle is applied. 
And we may thus hope to determine how the quantity of elec- 
tricity, or which amounts to the same thing, how the thickness 
of the electrical stratum varies upon different points of a body 
on which the electricity is not uniformly distributed. 

Take, for example, a conducting body of any figure what- 
ever, and place it upon an insulator, and, after having given it a 
certain portion of electricity, touch it with the small plane in a 
point «, cipable of being exactly determined ; place this plane 
in the balance, previously charged with electricity of the same 
kind, and observe the torsion necessary to counteract the repul- 
sion at a given distance D ; let this torsion be represented by A. 

Then withdraw the little plane, and touch the conducting 
body in another point a 7 , different from the former, but capable 
in like manner of being determined, and apply it to the balance, 
ascertaining the torsion necessary to bring the needle to the 
point D, as in the first experiment. Let this torsion be n A, its 
ratio to the first being expressed by n. If, after an interval of 
some minutes, we repeat these experiments, placing the little 
plane upon the same points «, a', we shall not find the same ab- 
solute torsions as before, because the insulated body will have 
lost a part of its electricity by contact with the air ; but the 
ratio of these torsions will remain the same. If the first becomes 
A\ the second will be n A', In order that the comparison may 
be perfectly exact, there should be the same interval between 
the successive contacts of a and a' as in the first experiment. 

We shall arrive at similar results, however often we may 
choose to repeat the experiment, and the ratio of the torsions will 



Electrical Equilibrium. ob 

continue the same as long as there remains a. sensible quantity 
of electricity upon the insulated body. Moreover, if we have 
noticed the times at which the successive experiments have been 
made, we shall see that the absolute decrease of the torsions is 
exactly such as ought to result from the simple contact of the 
air; in other words, the mutual repulsion of the small plane and 
the moveable circle, at any moment, is exactly the same as 
if we had left the plane in the balance with the charge of elec- 
tricity which it had taken from the point a or a', in its first con- 
tact. Consequently, the absolute quantity of electricity received 
at each contact, is proportional to the actual and total amount of 
electricity in the body. 

This proportion may also be made evident by the following 
experiment. 

30. Let the insulated body be a cylinder or a rectangular 
parallelopiped, the length being much greater than the breadth ; 
upon electrifying it and touching it with the little plane, first in 
the middle, and then at one of its extremities, we shall find the 
electrical action in these two cases to be very different. If we 
now bring to the electrified body, another of exactly the same form 
and dimensions, also insulated, and present it to the first symmetri- 
cally, that is, in such a manner that the similar sides shall come 
in contact, each throughout its whole extent, the electricity will 
of course be divided equally between the two bodies ; then, after 
separating them, if we repeat the experiment with the small plane, 
touching always the same points as before, we shall find that its 
electrical action is reduced, for each point, to exactly one half 
of what it was on the first trial. 

We see, therefore, from these experiments, that the absolute 
quantities of electricity, successively taken off by the trial plane 
from the same point of the surface of a conducting body, are pro- 
portional to the whole amount of electricity spread over the sur- 
face of this body at the instant of contact ; and, whatever may 
be this amount, that the quantities taken at the same instant from 
different points of the surface, preserve always the same invari- 
able ratios among themselves. Hence we draw tw r o conclusions ; 
the first is, that in every conducting body, the accumulation of a 
double, a triple, &c, quantity of electricity gives to each point 
of the surface, a double, triple, and generally, a proportiona/1 



36 Electricity. 

quantity ; the second is, that the trial plane, considered as infi- 
nitely small in relation to the whole surface of the conducting 
body, takes always at each point of the surface a quantity of 
electricity proportional to that of the point touched. 

Proceeding in this way, each contact of the plane diminishes 
somewhat the absolute quantity of electricity of the body which 
it touches, and consequently we ought, strictly speaking, to take 
account of this diminution, if we would bring our successive 
trials into exact comparison ; but this is rendered unnecessary 
by making the plane so small, that the quantity of electricity ta- 
ken off by it, shall be inconsiderable in comparison with that of 
the whole surface of the body. If, in addition to this precaution, 
we would reduce the error still more, we have only to carry 
back the plane to the surface of the body without discharging it. 
Care should be taken also to support the little planes by threads 
of very pure gum lac, having the greatest insulating power. 

31. As these experiments always require to be several times 
repeated, it is necessary, in comparing them with each other, 
to take notice of the loss of electricity occasioned by the con- 
tact of the air. This may be done according to the laws of de- 
crease above given ; but we may dispense with this correction, 
also, by combining the experiments in such a manner, that they 
shall rectify each other. For this purpose, if it is proposed to 
compare the electrical action of two points a and 6, we first 
touch a with the little plane, and observe the action which re- 
sults. We then touch 6, and observe the corresponding action. 
If there be a certain interval between the first and second obser- 
vations, as three minutes, for example, we should touch a again 
three minutes after the second observation, and take the arith- 
metrical mean between the two actions. We should thus have 
the same result as if the two contacts of a and b had been made 
at the same moment. This method of correction is to be prefer- 
red to any other. It even corrects the effects of loss along the 
supports, provided it is small, as it always is when they are 
well chosen and well prepared. 

32. To give an example of the method of alternate contacts, 
J shall select the following experiment, which I find among the 
manuscript minutes of Coulomb. 



Electrical Equilibrium. 



37 



He proposed to discover how electricity distributes itself upon 
a thin insulated plate. For this purpose, he insulated a plate of 
steel 1 1 inches in length, 1 inch in breadth, and t l T of an inch 
in thickness. In order to touch it through its whole breadth, he 
made a trial plane an inch in length and \ of an inch in breadth. 
He first applied this plane to the centre C of the plate, and af- F|g 1G 
terwards at an inch from the extremity, and he obtained the 
following results ; 





Observed 


Mean tor- 


Mean tor- 


Ratio of 




torsions. 


sions at the 


sions at 1 


mean tor- 






centre. 


inch from 
the end. 


sions. 


Contact at the centre. 


370° 








At 1 in. from the end. 


440 


360 


440 


1,22 


At the centre. 


350 


350 


417,5 


1,20 


At 1 in. from the end. 


395 


335 


395 


1,18 


At the centre. 


320 




Mea 


n...l,20 



That is, upon equal spaces, taken throughout the breadth of 
the plate at the centre, and at an inch from the extremities, the 
quantities of electricity are to each other as 1 to 1,2, and there- 
fore nearly equal. 

Coulomb repeated the experiment, placing the trial plane 
exactly at the extremity, but resting wholly upon the surface, 
and he obtained the following results ; 





Observed 


Mean tor- 


Mean tor- 


Ratio of 




torsions. 


sions at the 


sions at the 


the mean 






centre. 


end. 


torsions. 


Contact at the end. 


400° 








At the centre. 


195 


195 


395 


2,02 


jAt the end. 


390 


190 


390 


2,05 


At the centre. 


185 


185 


370 


2,0 


At the end. 


350 










Mean.... 2,02 



In this case the ratio of the quantities of electricity is much 
greater than in the preceding. Thus we see that while they are 
nearly constant from the centre to within one inch of each ex- 
tremity, beyond this the electricity increases very rapidly. 

Coulomb made a third experiment, placing the trial plane 
across the end of the plate at D so as to come in contact with 
both surfaces at once ; he then obtained the following results ; 



Electricity. 



Contact at the centre. 
At the edge. 
At the centre. 
At the edge. 



Observed 


Mean tor- 


Mean tor 


Ratio ofth^ 


torsions. 


ions at the 


sions at the 


mean tor 




centre. 


edge. 


sions. 


305° 








1175 


295 


1175 


3,98 


285 


285 


1156 


4,05 


1137 










Mean...4,01 



Fig. 11 



Thus the trial plane being placed across the end of the plate, 
receives just double the electricity which it acquired at this ex- 
tremity when it touched but one surface. 

The experiment being repeated with a plate 22 inches long, 
that is, of twice the length of the preceding, and otherwise of the 
same dimensions, gave exactly the same ratios between the in- 
tensities at the centre and at the extremities. 

33. Hence Coulomb infers; (1.) That in the contact upon 
the surface of the plate, the trial plane shares the electricity of 
only one of the two faces, wdiich is that to which it is applied ; 
(2.) That beyond a certain length of the plate, so considerable 
that the electricity shall be nearly uniform over the greater part of 
its surface, any prolongation has no sensible influence upon the 
ratios of the quantities of electricity accumulated at the extremi- 
ties and at the centre, the first being always double the second. 

To understand the full import of this remark, let AB be a 
plate whose length exceeds the limit just mentioned. Suppose 
the electrical state of the different points of its surface to be ex- 
amined, and represented by the ordinates CE, PM, QN r AA', 
BB'. These ordinates will not differ sensibly from each other 
until we arrive within about an inch of one of the extremities, 
after which they will go on rapidly increasing through the re- 
maining portion, so as to form the curve A'M or B'N. Now, 
since the ratio of AA' to PM or to CE is the same in all plates 
whose length is \ery great in comparison with their breadth, and 
as the same constant ratio obtains for the intermediate ordinates, 
it follows that the curve A'M or B' N preserves the same form for 
all these plates, and it is merely placed at the two extremities up- 
on the uniform lamina, whose thickness is CE ; and thus it is 
easy to foresee the electrical state of all plates of this description, 
when that at the centre is once known. 



Electrical Action at a Distance. 39 

This rapid increase of the electricity towards the extremity 
is not peculiar to plates ; it is found to take place generally in 
all elongated prismatic and cylindrical bodies ; and it is more 
rapid according as they are more thin. 

34. The tendency of electricity to the surface of conducting 
bodies, and the manner in which it distributes itself there, may 
be rendered evident by a very curious experiment. Let.^B F ;g j^ 
represent an insulated cylinder moveable about a horizontal 
axis, and made to revolve by means of the glass winch M, 
About this cylinder is wrapped a thin metallic sheet R, which 
terminates in a semicircle, and is attached to a cord of silk F. 
This apparatus is made to communicate with an electroscope, 
composed of two linen threads^/, supporting pith balls. The 
instrument being electrified, the threads /,/, diverge ; we then 
gradually unrol the sheet of metal, lifting it off by the cord F, 
and holding it suspended in the air. As it is extended, we see 
the linen threads approach, indicating a gradual diminution of 
action. If the sheet is sufficiently long, compared with the elec- 
trical charge given to the apparatus, the divergence will diminish 
till it becomes almost insensible ; but it will increase again, if, 
upon turning the winch M, the sheet of metal is again wrapped 
about the cylinder ; and then the action of the threads becomes 
the same as at first, allowance being made for the loss occasion- 
ed by the contact of the air. 



Of combined Electricities, and their Action at a Distance. 

35. We have thus far considered bodies as electrified by 
friction or communication. We come now to make known a 
class of phenomena in which the electrical state is produced 
without contact, and by the mere influence of electrified bodies 
at a distance. 

We take a cylindrical conductor J3, insulated in a horizontal f\« 13. 
position, the two extremities being hemispherical. We attach to it 
at small intervals linen threads, to which pith balls are suspend- 
ed. After touching this conductor, to make it certain that it is 
not charged with electricity, we bring it toward the electrified 



40 Electricity. 

body A, holding it by its insulating supports, and placing it al- 
ways at such a distance from A that the electricity cannot be 
communicated by a direct discharge. We shall then observe the 
following phenomena ; 

(1.) The threads placed at the extremities of the cylinder B 
diverge, and thus show that it is electrified. 

(2.) This divergence goes on diminishing toward the mid- 
dle of the cylinder, and there is a point at which there is no re- 
pulsive force whatever. 

(3.) This unelectrified point varies in its position upon the 
cylinder, according as it is moved from or toward the electrified 
body. 

(4.) If we present along the cylinder a pith ball, unelectrifi- 
ed and suspended by a thread of silk which insulates it, it is at- 
tracted throughout, except at the intermediate point of which we 
have just spoken. 

(5.) But if this pith ball be electrified, it is attracted by one 
extremity of the cylinder and repelled by the other, which 
shows that they are charged with different electricities. 

(6.) Indeed, if we touch these two extremities successively 
with a small insulated conducting body, and examine the elec- 
tricity thus obtained, we shall find that at the extremity next to 
the electrified body A, it is of a different nature from that of the 
body A ; and that it is of the same nature at the opposite ex- 
tremit}^. 

(7.) The signs of electricity cease if we remove the cylinder 
by its insulating supports, to a great distance from the electrified 
body A, or if we deprive the body A of its electricity. 

(8.) With the exception of this last case, the body originally 
electrified loses no part of its electricity by the influence which 
it exerts. No part of its electricity is transmitted to the cylin- 
der ; for if we measure its electrical action before the cylinder 
is brought toward it, and after it is withdrawn, we find that it 
has suffered no loss, except what is naturally due to the mere 
contact of the air. 

(9.) This constancy obtains only while it is beyond the in- 
fluence of the insulated cylinder. For while it is near an elec- 
trified body, if that be a conductor, the actioo at its surface is 
disturbed, as may be ascertained by means of the trial plane. 



Electrical Action at a Distance. 41 

(10.) If, without touching the electrified body A, we remove 
and replace the cylinder several times, the same phenomena 
will be repeated each time without any change, except what 
arises from the diminished intensity of the body A. 

The simple statement of these facts, leads us directly to the con- 
clusions to be derived from them ; (1.) Since the cylinder takes 
nothing from the electrified body, it must possess in itself the 
principles of the two electricities which are excited in it by the 
influence of this body ; (2.) Since these two electricities disap- 
pear when the influence of the foreign body ceases, although 
they cannot escape into the earth, we infer that their proportions 
are such that, being left to themselves, they mutually neutralize 
each other; (3.) Finally, this neutralization must needs take 
place without destroying them, for they reappear entire when- 
ever we expose the cylinder to the influence of the foreign body. 

36. We hence learn that the principles of the two electrici- 
ties exist naturally in all conducting bodies, in a state of combi- 
nation by which their effect is neutralized ; this we shall in fu- 
ture call the natural state of bodies. We now perceive that fric- 
tion, which seemed to be a means of creating them, serves only 
to disengage them from their combination, and to render one of 
them sensible by absorbing the* other; and this is the reason 
undoubtedly why we constantly observe that the rubbing body 
and the body rubbed exhibit contrary electricities. Finally, 
since the simple influence of an electrified body, presented at a 
distance, forces these two electricities to separate, and to arrange 
themselves so that those of a different nature are the nearest to 
each other, and those of the same nature the most remote, in 
enunciating this fact, we are compelled to admit, that opposite 
electricities attract^ and similar electricities repel each other r accord- 
ing to laws which we shall be able to determine by experiment. 
Thus, all the phenomena which we have described, become sim- 
ple, necessary, and evident consequences of this general property ; 
with the exception, perhaps, of one which may require some further 
elucidation. This is the momentary variation in the electrical ac- 
tion of the body A, while the cylinder is presented to it. But, since 
the free electricity upon the surface of one body acts upon those 
of other bodies, and destroys their combinations, at least in part, 
it is manifest that these electricities, being once set free, must in 

E. jr M. 6 



42 Electricity. 

their turn act upon the body which liberated them, and change 
the electrical action of the several points of its surface, either by 
causing the free electricty resident there, to distribute itself in a 
different manner, or by adding to this electricity that which the 
body is capable of furnishing by the decomposition of its natural 
electricity, or finally by acting in both these ways at the same 
time. 

37. These observations lead us to another important infer- 
ence ; in our first experiments, it may have been remarked that 
electrified bodies attract, or seem to attract, all light bodies which 
are presented to them, without its being necessary for this pur- 
pose to give them the electrical principle either by friction or 
communication. But w*e must now suppose that this excitement 
takes place of itself, by the simple influence at a distance 
of the electrified body upon the combined electricities of the 
light substances which are presented to it. Therefore in this 
case, the observed attraction, whether it be real or apparent, 
actually takes place only between electrified bodies. 

Moreover, the decomposition of the combined electricities is 
necessary in order that the attraction may manifest itself; for 
this attraction is so much the less lively according as the decom- 
position is more difficult, and ceases entirely if the decomposition 
be impossible. To be convinced of this, take two very fine threads 
of raw silk of equal length. Attach to them two small balls of 
equal dimensions, but of wbichone is of pure gum lac, and the other 
also of gum lac, but gilt or covered with tin-foil. These pendulous 
bodies being then placed at a small distance from each other, bring 
near them an electrified tube of glass or sealing wax ; we shall 
see that the ball covered with metal, upon the surface of which 
the decomposition of the combined electricities is easily effected, 
will be much more readily and strongly attracted than the other, 
which does not begin to discover signs of electricity till after a 
certain time, when the decomposition has at length taken place 
upon its surface ; and its electrical state continues even after it 
has been removed from the electrified body. The first ball, 
although covered with metal, also contracts in this way a durable 
electricity, because the resin it contains is impregnated with the 
electricity excited at the surface ; and both the one and the 
other are favoured in this respect by the contact of the air, 



Electrical Action at a Distance. 43 

which, on account of the influence of the electrified body, tends 
to take from each that one of the two combined electricities which 
is repelled by this body, while it has less power over the other, 
whose proper repulsive force is concealed by the attraction. Thus 
we remark generally, that insulated bodies which have been for 
some time submitted to the influence of an electrified body, come 
at length to have an excess of electricity of a nature opposite to 
that of this body, the effects of which appear after (hey are re- 
moved from its influence. 

As we shall have frequent occasion for the results at which 
we have now arrived, we shall reduce them to a sort of theorem ; 
thus, 

When an insulated conducting bo:Jy J5, taken in its natural 
state, is brought near to another body A, electrified and insulat- 
ed, the electricity upon the surface of «#, exerting its influence 
upon the two combined electricities of JB, decomposes a portion 
proportional to the intensity of its own action, and resolves this 
portion into its two constituent principles, repelling at the same 
time that of the same name with itself, and attracting that of the 
contrary name. The first withdraws to the part of the surface 
of B, which is most remote from A, while the second is collected 
on the part nearest to A. These two electricities having be- 
come free, act in turn upon the free electricity of A, and even 
upon its two combined electricities, of which a part is decom- 
posed by this reaction, and separated if the body A is also 
a conductor. This new separation produces a new decomposi- 
tion of the combined electricities of jB, and so on till the quanti- 
ties of each principle which have become free upon the two 
bodies, are put in a state of equilibrium by the balancing of all 
the attractive and repulsive forces exerted upon each other, in 
virtue of their different or similar nature. 

We shall soon inquire into the conditions according to which 
this equilibrium is determined. At present we suppose this state 
established ; and that we may continue to observe the develop- 
ment of the phenomena which result from it, we return to the in-*'ig 14. 
strument before used. Moreover, in order to render the state* 
ments as simple as possible, let us suppose that the electricity 
originally given to A is vitreous. Then if the conductor B is 
cylindrical, which we suppose, in order that the separation of 






44 Electricity. 

the combined electricities may be more manifest, the part R 
nearest to A will be in the resinous state, and the more remote 
part V in the vitreous. 

Things being thus disposed, we touch the part Fwith a third 
conductor C, also insulated and in its natural state, which, being 
withdrawn, will be found to be charged with vitreous electricity, 
the linen threads placed at V upon the conductor J5, collapsing 
at the same time, and those placed at R increasing their diver- 
gence. If, after this contact, we withdraw B from the vicinity 
of A, or if we touch A in order to deprive it of its electricity, we 
shall find B charged with resinous electricity only. 

This is a very simple consequence of the influence exerted 
at a distance. Before th."> contact, the vitreous electricity of Z?, 
crouded into the part J 7 , repelled the vitreous electricity of A i 
and attracted the resinous electricity developed in R ; it there- 
fore weakened the action of A upon R. By the contact of the 
third conductor C, we take away a portion of this electricity in 
V\ and the action of A upon i?, being less counteracted, becomes 
stronger. On account of this new increase of energy, there 
takes place in the conductor B, a new decomposition of the com- 
bined electricity, of which the vitreous part withdraws again to 
J 7 , and the resinous to it. Then the whole quantity accumulat- 
ed in R is necessarily greater than that in F", since this last was 
partially withdrawn by the contact of C, And thus, when we 
remove B from the influence of A, this vitreous electricity again 
becoming free, is not sufficient completely to neutralize the res- 
inous in i?, and we find the conductor B charged with an excess 
of resinous electricity. Owing to this inequality, the divergency 
of the threads, when under the influence of A, must be less in V 
than in i?, as from observation it is actually found to be. 

If we would carry this difference to the extreme, instead of 
touching the conductor B with an insulated body, which takes 
away only part of the electricity of J 7 , we touch it with an uninsu- 
lated body, and thus suffer it to communicate for a moment with 
the ground. Then all the electricity collected in V will escape ; 
and the threads suspended at this point will collapse entirely ; 
but the threads at R will" diverge still more than before, and we 
shall not diminish their divergency by touching again the ex- 
tremity V, But if we remove the conductor B from the influ- 
ence of A, the divergence will become much less. 



Electrical Action at a Distance. 45 

This also is easily explained. When we permit V to com- 
municate with the ground, all the electricity accumulated at 
this extremity, passes to the great mass of the earth, and its elec- 
trical action becomes insensible ; or, if you please, it decompos- 
es the combined electricity of the earth, attracts the resinous 
with which it is neutralized, and repels the vitreous which dis- 
tributes itself over the whole surface of the terrestrial globe. 
In whatever way we choose to consider it, there is no longer 
any free vitreous electricity in V, The vitreous electricity 
of A being now freed from this resistance, exerts a stronger 
attraction for R. This requires a new decomposition of the 
combined electricity of £, of which the vitreous part passes 
6ff, as before, to the ground, while the resinous is accumulated 
in R j and so on, till the attraction of A for the resinous electric- 
ity is completely satisfied. But these decompositions, which in 
our reasoning we have supposed successive in order to explain 
how they are effected, take place instantaneously in those metal- 
lic bodies, which may be regarded as perfect conductors ; and 
for this reason a single contact is sufficient to produce them com- 
pletely. After what has been said, it is evident why B, being 
removed from the influence of A, manifests an excess of resinous 
electricity, and why this excess is still greater than in the pre- 
ceding case. 

38. We have thus far confined ourselves to rendering evident by 
experiment the action of A upon B ; but we can also make the re- 
ciprocal action of B upon A manifest, either by touching the latter 
in different points of its surface with the trial plane, which would be 
the more exact way of proceeding ; or by simply suspending at the 
extremity of A, the most remote from B, linen threads terminat- 
ed with pith balls. We observe, in the first place, the diver- 
gence of these balls when the body A is insulated and removed 
from other bodies. Then, according as it is made to approach 
the conductor JB, and there takes place in this a decomposition 
of its combined electricity, the linen threads on A gradually 
collapse, because the vitreous electricity, residing in this part of 
A, withdraws toward £, till at length by the continued approach 
of A, the threads lose their divergence entirely, as if the body A 
were in its natural state ; and finally, there is developed in this 
part of A, a resinous electricity by the always increasing action 



46 Electricity; 

of R, when the threads are seen to diverge again, but with a 
different electricity. 

This succession of divergencies produced by contrary elec- 
tricities, with a natural state intervening between them, will be 
still more easily observed upon the conductor B, if, instead of 
presenting it to A in its natural state, we first give it a feeble 
resinous electricity ; for while it is removed from the influence 
of A, all the linen threads suspended from it, will diverge by 
reason of this electricity. But as B approaches A, and the 
action of A draws this resinous electricity toward the extremity 
of B nearest to A, we shall see the threads at the other extrem- 
ity gradually collapse, become parallel, and afterward diverge 
again in virtue of the vitreous electricity disengaged from its 
natural combination by the action of A, and repelled to this part 
of the conductor B. 

For the sake of distinctness, we have supposed that the body 
A is charged with vitreous electricity. But if it were charged 
with resinous electricity, all the phenomena would be precisely 
similar, and in the enunciation of the facts, it would only be 
necessary to change throughout the names of the two electrici- 
ties respectively. 

39. Having thus recognised generally the attractive and re- 
pulsive powers belonging to the two electricities, and having 
made known their natural state of combination in bodies, their 
separation by influence at a distance, and the general conse- 
quences which result from these properties ; we shall next en- 
deavour to subject these results to calculation, so as to be able to 
comprehend the facts enumerated in all their details, and to 
foretell, for instance, in the case of two bodies acting upon each 
other, the quantity and kind of electricity belonging to each 
point of their surfaces. 

But as we have found that the effects of these reciprocal in- 
fluences, so far as we have observed them, are exerted upon the 
electrical principles themselves, it is apparent that we shall not 
be able to trace them to their cause, except by determining the 
nature and manner of action of these principles; or, which to 
us is the same thing, by imagining from the observed phenome- 
na, some determinate mode of action which will exactly repre- 
sent the phenomena, and which admits of being verified, if not 



Electrical Action at a Distance. 47 

directly in its physical character, at least indirectly, but certain- 
ly, in its consequences. 

Now if we consider the extreme facility with which the 
two electricities diffuse themselves in conducting bodies, and 
tend to the surface, where they are retained by the pressure 
of the air ; if we consider also the perfect freedom with which 
they approach to or depart from each other, unite and separate 
without any loss of their original properties, it will be seen that 
the most probable view we can take of their nature is, to regard 
them as perfect fluids, the particles of which are impressed with 
attractive and repulsive powers, and which, in bodies where they 
can move freely, dispose themselves so as to be in equilibrium in 
virtue of all the interior and exterior forces which act upon them. 
40. It is easy to perceive that each of these fluids must possess 
in itself a cause of repulsion by which its particles are driven from 
each other ; for if we suppose a certain quantity of vitreous or res- 
inous electricity to be introduced into a metallic sphere where its 
motions are free, we know that it will tend entirely to the surface 
where it will form a very thin stratum. If we augment the diam- 
eter of the sphere, the electrical stratum will retire farther and 
farther from the centre, diminishing always in thickness ; finally, 
if we remove the pressure of the air entirely, the electricity will 
be completely dissipated. These effects certainly indicate a 
repulsive action exerted between electrical particles of the same 
nature ; and all the phenomena in which the two combined elec- 
tricities are separated from each other by influence at a distance, 
perfectly confirm this result, while they also demonstrate the 
existence of a reciprocal attraction between electricities of a 
different nature. 

But the experiments which we have now related for the pur- 
pose of establishing the mutual repulsion of electric particles 
of the same nature, make known another important property, 
namely, the incompressibility of the electric principle, on the 
supposition that it is a fluid. For, if it were compressible, like 
the air, for example, when it is diffused through a conducting 
body, the mere effect of its own repulsive force would undoubt- 
edly cause the greater part to flow to the exterior surface, where 
it would be condensed by the repulsion from within ; and thus 
the density would go on gradually diminishing from this surface 



48 Electricity. 

towards the interior of the body ; but the inner strata, however 
much we suppose them to be rarified, would yet, strictly speak- 
ing, never cease; and thus we should always find electricity 
within the body in greater or less quantity, whereas we do not, by 
the most delicate tests, discover the least trace of it. It follows, 
therefore, that in order to reconcile this fact with the nature of 
the electric principle, we must suppose it incapable of being sen- 
sibly compressed ; and different quantities being successively 
introduced into the same conducting body, and diffusing them- 
selves there and flowing to its surface, must cause the electric stra- 
tum, situated at the surface, to take different thicknesses, which 
are always infinitely small, at least in all states to which we are 
able to reduce it. 

41. We find also from the same phenomena, that these at- 
tractions and repulsions diminish in force as the distance increas- 
es ; but according to what law ? Among the different laws which 
may be supposed to exist, there is one which perfectly rep- 
resents all the phenomena; namely, that which supposes the 
force to vary in the inverse ratio of the squares of the distances. 
Adopting this law, the essential properties of the two electric prin- 
ciples are comprehended in the following proposition ; each of the 
two electric principles is an incompressible fluid, the particles of which, 
possessing perfect mobility, mutually repel each other, and attract those 
of the opposite principle, with forces varying in the inverse ratio of 
the squares of the distances. Moreover, at equal distances, the at- 
tractive force is equal to the repulsive ; this equality is necessary 
in order that the two combined electricities in unelectrified bod- 
ies may exert no action at a distance, as may be proved by ex- 
periment ; take two discs of thin glass AB, A'B', whose surfaces 
Fi<*. 15. are g rou nd very true, and which are about 4 inches in diameter ; 
to each of them fix a handle CM of glass or sealing wax or any 
other insulating substance ; then, having prepared a very sensible 
pendulum, consisting of a small pith ball suspended by a fibre of 
silk, as it comes from the silk worm, rub the discs against each 
other, holding them by the insulating handles ; and without sep- 
arating them, present them together to the pendulum. You will 
see that they exert upon it no attraction ; but separate them and 
present them to it in turn, and they will each attract it. They 
are therefore mutually electrified by the friction ; and one has 



Electrical Action at a Distance. 49 

acquired the vitreous and the other the resinous electricity, as 
may be proved by presenting them to a second very sensible 
pendulum, charged with a known electricity. But these elec- 
tricities do not become manifest when the discs are in contact, 
for residing upon the two surfaces in contact, the distances of all 
their points from the pendulum is absolutely the same, and there- 
fore the opposite actions which they exert to separate the com- 
bined electricities of the ball are equal ; and thus their total re- 
sultant is nothing. We may also modify this experiment so as 
to make the compensation of forces progressive. For this pur- 
pose, after having separated the discs, we bring the electrified 
surface of one of them in contact with the pendulum. After 
the ball has taken the small quantity of electricity proportional 
to its magnitude, it is repelled. Keeping it in this state of re- 
pulsion, we present the other face of the disc, as represented in 
figure 16; (for the electricity will act upon it with the same 
power through the thickness of the glass.) Then bring the 
second disc gradually toward the first, in the manner represent- 
ed in the figure. As they are made to approach each other, we 
shall see the repulsion diminish and the small pendulum descend 
more and more, and finally, when ihe discs come into actual 
contact, they will together produce no effect upon the pendulum, 
but it will be driven off again when they are separated. These 
two electricities thus neutralized by their contact, represent 
to us the natural state of the combined electricities, with this 
difference only, that in conducting bodies, the two electric- 
ities are united to each other simply by their force of combina- 
tion, and may be separated by the action at a distance of a free 
electricity ; while in the glass discs, each of the electricities is 
retained by the resistance which the nonconducting nature of the 
glass opposes to the freedom of its motions. For this reason, 
the experiment which we have now described would succeed 
equally well with discs of gum lac or sealing wax, or even with 
one disc of a substance of this kind and one of metal. But it 
could not take place with two discs of a conducting nature ; for 
then no resistance being opposed to the motion of the electrici- 
ties, they would unite and combine anew as fast as they were 
disengaged by the friction. 
E. & M. 7 



50 Electricity. 

42. Having thus distinctly defined the -character and mode 
of action of the two fluids, we proceed to consider the mathemat- 
ical consequences to be deduced from this definition, for the pur- 
pose of comparing them with the observed phenomena and noting 
the agreement or disagreement. It is especially incumbent on 
us to point out those consequences which are susceptible of a 
precise and numerical determination, and which accordingly 
admit of being rigorously verified. But this would lead us into 
very profound calculations, requiring all the resources of analy- 
sis ; and with all these resources, it was not till very lately that 
the problem was solved in an exact and general manner. This 
fine discovery is due to M. Poisson. We shall be indebted, 
therefore, to his labours for the precise results with which the 
calculus has made us acquainted ; we shall take them as rigo- 
rous deductions from our first definitions, and it will only remain 
for us to determine whether they agree with the facts. 

43. Let us begin with considering a single conducting insu- 
lated body, charged with an excess of vitreous or resinous elec- 
tricity, and withdrawn from all foreign influence. 

According to the calculation, the fluid introduced into this 
body, will tend entirely to its surface, and will form there an 
extremely thin stratum. This is confirmed by the most minute 
and exact observations. 

Again, the calculus determines the interior surface of this 
stratum and its thickness. The exterior surface, being confined 
by the air, is the same as that of the body. In this case, the 
air is to the free electricity, like an impermeable vessel of a given 
form, which contains the electricity within itself, and resists by its 
pressure the efforts of the fluid to expand. The inner surface can 
differ but little from the outer, because the stratum is very thin. 
But in order that the body may remain in a permanent electric- 
al state, the form of this surface must be such that the entire 
stratum may exert neither attraction nor repulsion upon the 
points comprehended within its cavity ; for if any action did 
-exist, it would be exerted upon the combined electricities of the 
body, and decompose a part of them, and consequently the 
electrical state of the body would be changed. The analytical 
condition which establishes this property, determines also the 
form and thickness of the stratum, which may, and in general 



Electrical Action at a Distance, 5 1 

must be unequal upon the different parts of the surface of the 
electrified body. If this body has, for example, the form of a 
sphere, the two surfaces of the electrical stratum will be spherical, 
and will have their centre at the centre of the sphere. The 
thickness of the stratum will therefore be constant throughout, and 
equal to the difference of their radii. Indeed it is demonstrated 
that, according to the law of the squares of the distances, such 
a stratum exerts no action upon the points within it. 

44. If the figure proposed be an ellipsoid, the inner surface of 
the electrical stratum will be that of a concentric and similar ellip- 
soid, for it is demonstrated that an elliptical stratum, the surfaces 
of which are thus concentric and similar, exerts no action upon 
a point situated within. The thickness of the stratum at each of 
these points is generally determined by this construction ; and 
it hence results that this thickness is greatest at the extremity 
of the transverse axis, and least at the extremity of the conjugate ; 
and that the thicknesses which answer to the two different ex- 
tremities are to each other as the axes. 

In all these cases, the exterior surface of the fluid stratum is 
determined by the surface of the body, and the question is re- 
duced to finding for the inner surface a figure but little different 
which shall reduce to nothing the whole action of the stratum 
upon the several points comprehended within it. 

45. These different results do not admit of being directly 
subjected to experiment, but they are connected with others 
which may be verified, and which we shall soon make known* 

The electrical stratum, being disposed as we have said, acts 
by attraction and repulsion upon the other electrical particles 
situated without the exterior surface or in this same surface. It 
attracts them if they are of a different nature from itself, and 
repels them if they are of the same nature. The latter case is 
that of the electric particles which form the exterior surface of 
the stratum ; each one of them is repelled from within outward 
with a force proportional to the thickness of the stratum at this 
point. The particles situated below the surface, in the thickness 
of the stratum itself, suffer a similar but less repulsion, because it 
is proportional simply to the thickness which separates them from 
the inner surface, and because the particles which enclose them 
on the side of the exterior surface exert upon them no action 



52 Electricity. 

whatever. All these gradually decreasing, repulsive forces, be- 
ing resisted in their action by the exterior air which opposes the 
escape of the electric particles, there must hence manifestly re- 
sult, upon the whole, a pressure exerted against the air and tend- 
ing to repel it. This pressure is in the compound ratio of the 
repulsive force exerted at the surface and the thickness of the 
stratum ; and as one of these elements is always proportional to 
the other, we may say that it is, at each point, proportional to 
the square of the thickness ; it must, therefore, in general be 
variable upon the surface of electrified bodies. If this pressure 
is throughout less than the resistance opposed by the air, the 
fluid is retained in this vessel of air, and cannot escape. But 
if the pressure, at certain points of the surface, prevails over the 
resistance of the air, then the vessel is broken and the fluid es- 
capes as through an orifice. This takes place at the extremities 
of points, and upon the acute edges of angular bodies ; for it may 
be demonstrated that at the vertex of a cone, for instance, the 
the pressure of the electric fluid would become infinite, if the 
electricity could accumulate there. At the surface of an oblong 
ellipsoid of revolution, the pressure is not infinite at any point ; 
but it will be greater at the two poles, according to the ratio of the 
axis which connects them to the diameter of the equator. Agree- 
ably to the theorems which have now been cited, this pressure 
will be to that which takes place at the equator of the same body, 
as the square of the polar axis is to the square of the equatorial 
diameter ; and thus if the ellipsoid be very much elongated, the 
pressure may be but feeble at the equator, while at the poles it 
exceeds the resistance of the air. Thus when we electrify a 
metallic bar which has the form of a very elongated ellipsoid, 
the electric fluid tends principally toward its extremities, and 
escapes at these points, in virtue of its excess of pressure 
over the resistance of the opposing air. Generally, the indefi- 
nite increase of the electric pressure in certain parts of elec- 
trified bodies, furnishes a natural and exact explanation of the 
power possessed by points of rapidly dissipating in the noncon- 
ducting air the electric fluid with which they are charged. 

If the nature of the electrified body be such as not to admit 
of the free motion of its electricity, the excess of pressure of 
which we have now spoken, would be exerted against the par- 



Electrical Action at a Distance, 53 

tides of the body which might envelop the electric stratum ; or, 
generally, against those which either by their affinity, or by any 
other mode of resistance, should oppose its dissipation. 

46. Having determined, from theory, the manner in which 
the electric fluid disposes of itself in a single conducting body, 
insulated and withdrawn from all foreign influence, we pass to 
the more complicated case in which several electrified conduct- 
ing bodies exert a mutual influence upon each other ; and, as 
it is necessary to select bodies whose form renders the phenom- 
ena accessible to the calculus, let us consider two spheres of 
conducting matter, both electrified and brought within a small 
distance of each other. 

47. The distribution of the electricity in this case, and in all 
cases where several electrified bodies act mutually upon each 
other, depends on a general principle which is self-evident, and 
is moreover attended with the great advantage of reducing di- 
rectly all these questions to a mathematical condition. We 
here give its enunciation in language borrowed from the interest- 
ing work of M. Poisson. 

" If several electrified conducting bodies be brought within 
the influence of each other, and come to a permanent electrical 
state, it is necessary that the resultant of the actions of the elec- 
tric strata which cover them, upon any point taken in the inte- 
rior of one of these bodies, should be zero. For if this resultant 
were of any value whatever, the combined electricity residing 
in the point in question, would be decomposed, and the electric 
state would be changed, contrary to our supposition of its being 
permanent." 

This principle, expressed by a formula, furnishes immediate- 
ly as many equations as there are bodies to be considered, and 
unknown quantities presented by the problem ; but the solu- 
tion often eludes the power of analysis. Nevertheless M. Pois- 
son, who had so happily possessed himself of the general key 
to this theory, has succeeded in removing all the analytical dif- 
ficulties for the case of two spheres brought into contact or with- 
in the influence of each other, and originally charged with any 
quantity whatever of electricity. The formulas at which he 
arrived offer a great number of results which may be verified by 
experiment, and which are so many rigorous proofs of the truth 



54 Electricity. 

of the theory. I shall confine myself to citing a single instance, 
the particulars of which are very remarkable. It takes place 
when two spheres of unequal diameters, after having been brought 
into contact and simultaneously electrified, are gradually remov- 
ed from each other to different distances remaining insulated all 
the while. Their electrical state is found to undergo the most 
singular changes. In the first place, at the moment of contact 
the electricity being examined with the trial plane, is found to 
be of the same nature on the two spheres, as was to be expected ; 
but it is nothing at the point of contact. Now if we separate the 
two spheres, their dimensions being, according to our supposition, 
unequal, there is no point destitute of electricity. The natural 
electricity of the smaller sphere is decomposed, and that which is of 
a contrary nature to the electricity of the larger sphere, tends to- 
ward the point where the contact took place. This effect dimin- 
ishes as the spheres recede, and entirely disappears at a certain 
distance, depending on the ratio of their diameters, when the 
point of the small sphere where the contact took place, is found 
to be in its natural state ; and finally at a still greater distance, 
this point is covered with electricity like the rest of the sphere 
of which it is a part. These singular alternations, the distance 
at which they occur, their constant appearance on the surface 
of the smaller sphere, may all be determined by the trial plane, 
and they may all likewise be predicted with the same precision 
by means of the formulas of M. Poisson. 

48. Not being able to enter here into more minute verifi- 
cations, we shall take them for granted, and proceed to give from 
theory a precise definition of the several particulars of electrical 
action which are often confounded. 

The first thing to be considered in electrical experiments, is 
the nature of the fluid which is found to reside on the surface of 
bodies, and on each point of this surface ; it is determined by 
eontact with the trial plane, which is presented to the needle of 
the electroscope previously charged with a known electricity. 

The second thing is the quantity of electricity accumulated 
at each point, or, which amounts to the same thing, the thickness 
of the electric stratum. This is also measured by touching the 
point in question with the trial plane and presenting it to 
the needle previously charged with the same electricity. The 



Electrical Action at a Distance, 55 

force of torsion necessary to counterbalance the electric action 
of the plane, is, at equal distances, proportional to its quantity of 
electricity, or, which amounts to the same thing, to the thickness 
of the electric stratum, upon the superficial point which it has 
touched. 

The third thing to be considered theoretically, is the influ- 
ence, exerted by each element of the electric stratum, upon a 
particle of the fluid situated in the exterior surface or without 
this surface. The attraction or the repulsion thus considered, 
is directly proportional to the thickness of the electric stratum 
at the superficial point which attracts or repels, and it is in- 
versely as the squares of the distances which separate this point 
from the point attracted or repelled. 

Finally, the last thing to be considered is the pressure which 
the electricity exerts against the exterior air at each point of 
the surface of the electrified body. The intensity of this pres- 
sure is proportional to the square of the thickness of the electric 
stratum. 

Regard being had to these particulars, we shall be in no 
danger of erring by vague considerations ; and if, at the same 
time, we take into the account the decomposition of electricity 
by influence at a distance, we shall be able to explain almost any 
electrical phenomenon that can occur, 

49. We here remark, that whatever be the real nature of the 
electric principle, since the constitution which we have attributed 
to the two fluids gives rise to almost all the phenomena in number 
and kind which have been deduced by calculation, there is suffi- 
cient reason for admitting this constitution provisionally in our 
subsequent inquiries ; for, from the proofs already given, we 
may affirm, that whatever be the actual nature of the electric 
principle, it must adapt itself to the same facts with the same 
exactness, and consequently must be susceptible of the conditions 
we have attributed to the two fluids, so that the facts may hence 
be deduced in a similar manner, and by similar formulas with 
those above employed. But new observations or new ap- 
plications will serve, in a more advanced state of the calculus, 
to confirm or refute this theory, and to show whether it is the 
exact and general interpretation of all the phenomena, or merely 
the approximate and particular expression for those which have 
been hitherto submitted to it. 



56 Electricity. 

A sensible progress may be observed already in the succes- 
sion of theories, preceding that now proposed, among which 
there is one which has been too much celebrated, and in fact too 
useful to be passed over in silence. Most electrical phenomena, 
if we confine ourselves to their general circumstances, may be 
explained on the supposition of a single fluid, diffused in a certain 
quantity through all bodies, and forming their natural state. An 
excess of this fluid is what we have called the vitreous electric- 
ity, and a deficiency, what we have called the resinous ; hence 
result two states of bodies, which the advocates for this system 
designate by the names of positive and negative. They admit 
also that the particles of the electric fluid mutually repel each 
other. But since experiment shows that bodies in their natural 
state exert no electric action upon each other, they are obliged 
to suppose that the electric particles are attracted by the proper 
matter of bodies. In fine, it has been shown by a thorough and 
rigorous investigation, that this hypothesis will not account for 
an equilibrium, and that it would, moreover, be necessary to 
suppose the particles of bodies to exert upon each other a re- 
pulsive action, sensible at great distances, like the electric influ- 
ence itself, and varying with the distance according to the same 
law. Franklin, the author of this theory, employed it very in- 
geniously in explaining all the electric phenomena known in his 
time, and which till then remained insulated and scattered ; but he 
did not perceive the paradoxical consequence to which his hy- 
pothesis led. iEpinus was the first, who, by an exact analysis 
of all the forces which concur in producing the electric equilib- 
rium, discovered the necessity of a repulsion between the mate- 
rial particles of bodies ; t after tym the celebrated Henry Cav- 
endish was also led to the same conclusion ; for he made this 
repulsion one of the essential conditions of an hypothesis respect- 
ing the nature of electricity, which he published in the Philo- 
sophical Transactions for the year 1771, and which is very sim- 
ilar to that of ^pinus. 

Although such a repulsive force between the material parti- 
cles of all bodies may, at first view, seem absolutely incompati- 
ble with the more general phenomena of the universe, and par- 

t Tentamen Thcoricc Electricitatis et Magnetismi, p. 39. 



Electrkal Action at a Distance, 57 

ticularly with the law of the celestial attraction, yet it is not so 
in reality. For this repulsion, as iEpinus and Cavendish employ 
it, would be exactly counterbalanced by the mutual attraction 
which their hypothesis supposes to exist between the particles of 
matter and those of the electric fluid diffused through all bodies 
in their natural state ; so that in fact, these two contrary causes 
would produce no effect upon bodies while in this state, and they 
would in no way impair the effects of the universal attraction 
which is exerted between bodies, independently of their elec- 
tricity. Now admitting such a state of things, most electrical 
phenomena may be accounted for, and their mutual dependence 
conceived, and even foretold, not indeed particularly and numer- 
ically, but as to their general character. We may thus explain, 
for instance, the mutual attraction and repulsion of electrified 
bodies, and even the development of the electric properties of 
bodies in their natural state, by the mere influence at a distance 
of another electrical body. But since the time when this theory 
was first proposed, many particulars have been observed more 
in detail and fixed with more precision. A great number have 
been rigorously determined. We have the results, indeed, in 
numbers ; and it is these numbers that we are to represent. For 
example, when iEpinus and Cavendish wrote, the law of electric 
attraction and repulsion had not yet been established by actual 
experiment ; it might accordingly be doubted whether the forces 
which produced these phenomena varied as the square, the cube, 
or any other power of the distance. It was therefore impossible 
to determine numerically the distribution of electricity in bodies> 
when it comes to a state of equilibrium, in virtue merely of its 
action upon itself; or to assign the proportion of its division be- 
tween two bodies of a given form, since these delicate phenom- 
ena depend on the law according to which the fluid acts upon it- 
self and upon other bodies. According to the law of the cube, 
for example, the conditions of distribution and division of the 
electricity would be very different from what they are according 
to the law of the square ; therefore we may now reject the first 
of these laws as contrary to the phenomena. In the same way, 
if we should introduce the law of the square of the distance into 
the hypotheses of Cavendish or iEpinus, we should probably be 
led to consequences contrary to the actual relations of distribu- 
E.irM. 8 



58 Electricity, 

tion and division which we now know by observation; whence 
we should infer also the fallacy of this hypothesis. This deduc- 
tion has not yet been made ; and from the complicated nature of 
the hypothesis, it would seem that the problem is a very difficult 
one. Happily it is of no great importance ; for the hypothesis 
being thus followed out into its consequences, could only be 
found, on the supposition of the most favourable result, to agree 
with the phenomena ; and this agreement is already known to 
exist in the case of the theory of two fluids, and with the 
most perfect exactness, and what is of no small importance, with 
simplicity, a complete analogy, and under a form which renders 
it susceptible of being easily subjected to the calculus. 



Theory of the Motions produced in Bodies by Electric Attraction 
and Repulsion, 

50. At the outset of our inquiries into electrical phenomena, we 
discovered that two electrified bodies, when placed at a certain 
distance asunder, seem to attract or repel each other. It appeared 
afterwards that the attraction and repulsion take place only be- 
tween the two electric fluids, and that the substance of the bodies 
does not, by any law of affinity, partake of these motions. It 
becomes necessary, therefore, to examine how, and by what me- 
chanism, these forces are transmitted to the substance of bodies, 
and made to produce in them the motions which we observe. 

For the sake of simplicity, we shall confine ourselves, in the 
first place, to the consideration of two electrified spheres A and 
B, the one A fixed, the other B moveable. Three cases may be 
supposed which it will be necessary to consider separately. 

(1.) A and B non-conductors; 

(2.) A a non-conductor, B a conductor ; 

(3.) A and B conductors. 

51. In the first case, the electric particles are fixed upon the 
bodies A and B by the unknown force which is the cause of 
their non-conducting property. Not being able to quit these 
bodies, they communicate to them the motions which their re- 
ciprocal action tends to impress upon themselves. 



Theory of Electric Attraction and Repulsion. 59 

The forces, then, by which motion may be produced, are ; 
(1.) The mutual attraction and repulsion which the fluids of A 
and B exert upon each other; (2.) The repulsion of the fluid 
of B for itself. But as the mutual repulsion of the parts of a sys- 
tem can produce no motion in its centre of gravity, the effects of 
this latter action destroy each other upon the two spheres re- 
spectively, and no motion can result from it of one toward the 
other. We need take account, therefore, only of the first kind 
of forces. If the distribution of the electricity be uniform upon 
each sphere, each will attract or repel the other just as if its 
whole electrical mass were united at its centre, and the whole 
force of attraction or repulsion is proportional to the product of 
the whole quantity of electricity which they possess. This 
force transmits itself to the ponderable matter of the two spheres 
A and 23, in virtue of the adhesion by which they retain the 
electric particles ; and, on account of the two factors of which 
its expression is composed, it will be seen that it would become 
nothing if one or the other of the two spheres were not first 
charged with a foreign electricity. During the motion, it suffers 
no variation except that which arises from change of distance, 
because the two spheres, being supposed to be of substances strict- 
ly non-conducting, their reciprocal action can produce no new 
development of electricity. 

52. In the second case, the sphere 22, supposed to be of con- 
ducting matter, suffers a decomposition of its natural electricities 
by the influence of A. The opposite electricities w r hich result 
from this decomposition unite with the foreign electricity com- 
municated to this sphere, and they arrange themselves together 
agreeably to the laws of electric equilibrium ; then the motion of 
B toward A may be considered in two points of view. 

Let us suppose, in the first place, that without disturbing the 
electrical state of B, we spread over its surface an insulating 
wrapper, solid, without weight, and adhering to it throughout. 
The electricity of B, being unable to escape, w 7 ill press upon the 
wrapper, and by this means transmit to the particles of the body 
the forces by which it is itself acted upon. Then the forces 
which act upon the system will be, (1.) The mutual attraction 
or repulsion of the fluid of A and the fluid of B ; (2.) The re- 
pulsion of the fluid of B among its ow T n particles; which, howev- 



60 Electricity. 

er, can produce no motion in the centre of gravity of B-, (3.) 
The pressure of the fluid of B upon the insulating wrapper ; but 
this pressure is exactly counterbalanced by the reaction of the 
wrapper, and no motion can result from it. The first force 
therefore, is the only one which we need consider. 

When the distance D of the two spheres is very great com- 
pared with the radii of their surfaces, the decomposed electrici- 
ties of B are distributed, according to calculation as well as ex- 
periment, nearly equally upon the hemisphere situated toward 
A and that opposite to A. Then the actions which they expe- 
rience from A are nearly equal and destroy each other. The ef- 
fective force, therefore, results wholly from the quantities of foreign 
electricity communicated to the two spheres, and it is proportion- 
al to the product of these quantities. So long as the spheres are 
at a great distance from each other, this product and the attrac- 
tive or repulsive force which it measures, vary only on account 
of the change of distance. But this is an approximation. For, 
strictly speaking, the electrical state of B varies as it approaches 
A, on account of the decomposition of its natural electricity pro- 
duced by this sphere. Consequently the reciprocal action of 
the two spheres must also vary in a very complicated manner. 
The supposition of an insulating wrapper without weight, 
serves here only to connect the electric fluid with the material 
particles of the body B. This supposition may be considered as 
realized by the thin layer of air by which bodies are surrounded, 
and which adheres to their surface. But we may arrive at the 
same result in another way ; in this case, it is necessary to con- 
sider the pressures produced upon the air by the electricities 
which exist in B in a state of freedom. In fact, these elec- 
tricities, as well those which have been communicated to 
the body, as those which have been decomposed there by 
influence, tend toward the surface of J5, where the air arrests 
them by its pressure and prevents their passing off. They dis- 
pose of themselves under this surface, therefore, in the manner 
required by their mutual action and the influence of the body «#, 
supporting themselves against the air which prevents their expand- 
ing. But reciprocally, they press the air from within outward, 
and tend to lift it up with a force which is proportional to the 
square of the thickness of the electrical stratum at each point. 



Theory of Electric Attraction and Repulsion, 61 

Decompose all these pressures according to three rectangular Mech 
co-ordinates x, y, z, one of which z is directed toward the centre 73. 
of the sphere A, and take their partial sums ; we shall find that 
according to x and */, they are nothing, so that there finally re- 
mains only one resultant directed toward the centre of the sphere 
A. When the spheres are at a great distance from each other, 
compared with the radii of their surfaces, the decomposed elec- 
tricities of B press the exterior air in contrary directions with 
an intensity nearly equal, and their effects almost exactly des- 
troy each other. There remains, therefore, only the effect of 
the foreign quantities introduced into the two spheres; and 
there results from it an excess of pressure directed according to 
the line of the centres, and proportional to the product of these 
quantities, that is, exactly equal to what the other method gave. 
It is evident, moreover, that this expression is subject to the 
same limitation, since the pressures produced by the electrical 
stratum against the exterior air must vary with the quantity of 
natural electricity decomposed in B by the influence of A, ac- 
cording as the two spheres approach each other. 

53. The third case, where A and B are both conductors, is 
resolved upon precisely .the same principles, either by imagining 
the two electrified surfaces covered with an insulating wrapper, 
and calculating the reciprocal actions of the two fluids which 
transmit themselves by this means to the material particles of 
the body ; or by considering the pressures produced upon the 
exterior air by the two electrical strata, and calculating the ex- 
cess of these pressures according to the line which joins the two 
centres. Only, in this case, the attractive or repulsive force of 
the two spheres will vary, according as they approach each 
other, not only on account of the consequent difference in the 
intensity of the electrical action, but also by the progressive 
decomposition of their natural electricities which will take place 
in the two conducting bodies A and B. 

The results to which we have now arrived would still hold 
true if the spheres A and B were both free to move toward each 
other ; for without disturbing their reciprocal action, we may 
always impress on either of them its motion in a contrary direc- 
tion, and this would reduce it to a state of rest, and refer the 
problem to the case which we have considered. We have taken 



62 Electricity, 

bodies of a spherical form, because we are able to perform the 
calculations which give, in each case, the values of the attrac- 
tion. The same reasoning will apply equally to all cases of 
attraction. 

54. Let us consider, for example, the phenomena which are 
presented by an electrical pendulum drawn from a perpendicu- 
lar by the action of an electrified tube. For the sake of distinct- 
ness, let us suppose this pendulum to be formed of a small pith 
Fig. 17. ball suspended by a thread of silk CS, and charged with vitreous 
electricity. As long as the ball is withdrawn from all foreign 
influence, the electricity will dispose of itself under the surface 
in a very thin spherical stratum, of an equal thickness through- 
out ; and consequently, the pressure which it will exert upon 
the exterior air will be equal also throughout, since it is at each 
point proportional to the square of the thickness of the stratum. 
The ball will therefore be less pressed by the exterior air than 
if it had no electricity at its surface, but it will be equally so 
throughout, and consequently will have no motion in any direc- 
tion. 

Suppose now that at some distance from its surface, a tube of 
gum lac or sealing wax is presented, electrified resinously ; a 
portion of the natural electricities of the ball will be immediately 
decomposed. The resinous part will recede from the tube, and 
the vitreous part will tend toward it. This last motion will take 
place also in the foreign vitreous electricity, which was at first 
spread beneath the surface of the ball. The pressure upon the 
air, which is always proportional to the square of the thickness 
of the electrical stratum, will be most powerful on the side to- 
ward the tube ; and consequently the atmospheric pressure, which 
was before equal over the whole surface, will become compar- 
atively more powerful on the opposite side. This excess of press- 
ure will therefore urge the ball toward the resinous tube ; and 
if we wish to retain it in its place by another thread of silk CS', 
acting in the direction opposite to this tendency, CS' will sustain 
all the effort produced by the difference of pressure. 

Let us suppose now that the thread is cut. The ball will 
yield to the force exerted upon it, and the insulating thread CS 
which supports it will be drawn from a perpendicular position. 
But this deviation will have a limit ; for the weight of the ball, 



Theory of Electric Attraction and Repulsion. 63 

which, in its first position, was supported by the point of suspen- 
sion S, is only partially supported by it in the oblique position 
SO. Indeed, if we represent the effort of this weight by the lg ' 
vertical line OP, we may decompose it into two other forces, one 
C'Q in the direction of the thread produced and which is des- 
troyed by the resistance of this thread, the other OR perpen- 
dicular to the thread and tending to bring back the ball to the 
lowest point. Now this second force will evidently increase with 
the angle CSO ; and consequently it will tend so much the more 
to make the ball descend as it is farther removed from a perpen- 
dicular. Consequently, in each position of the tube, the devia- 
tion of the thread will be such, that the excess of atmospher- 
ic pressure, tending to make it rise, shall be equal to the decom- 
posed gravity which tends to make it descend. 

55. We have supposed the tube and the ball to be charged 
with opposite electricities ; if the electricities were of the same 
nature, they would repel instead of attracting each other. The 
pressure of the electricity of the ball against the exterior air 
would be greatest on the part most distant from the tube, and 
accordingly it would make an effort to depart from the tube. 

56. We have thus considered what generally takes place ; 
but in certain cases a phenomenon occurs which appears at first 
view entirely to contradict the above reasoning. On bringing 
two bodies, similarly electrified, toward each other, the repulsive 
force is found to diminish, and, the bodies being brought still 
nearer, it is finally changed into attraction. This takes place 
ordinarily when one of the bodies is very small compared with 
the other and feebly electrified ; for example, in the case where 
the pith ball of the electrical pendulum, is charged with resin- 
ous electricity, and a large tube of sealing wax, also electrified 
resinously, is gradually brought nearer and nearer. But far 
from being an exception to our theory, this phenomenon is in fact 
a consequence of it. In proportion as the tube, on the approach 
of the ball, repels the resinous electricity which was first given 
to it, it decomposes a much greater part of its combined electric- 
ities. It repels the resinous which goes to join that first given 
to the ball, and attracts the vitreous toward itself. If there 
were only these two decomposed electricities on the surface of 
the ball, it would evidently be attracted toward the tube ; and 



64 Electricity. 

this attraction would increase as the distance diminished, and : 
the tube became more highly electrified ; and there would be 
• no limit to this increase of attraction. But it is not so with 
the repulsion which, on account of the quantity of resinous 
electricity at first given to the ball, can increase only with the 
diminution of the distance. If, therefore, its force at a certain 
distance is less than the attraction owing to the progressive de- 
velopment of the combined electricities, the latter force will pre- 
vail, and the ball will approach the tube. We thus perceive 
that the phenomenon depends on the relative proportions of 
electricity at first given to the ball and tube ; and without being 
able to assign these proportions, we see that the change from 
repulsion to attraction will take place the more readily and at 
a greater distance, according as the tube has more electricity 
and the ball less ; and thus, if the distance is fixed, the repul- 
sion and attraction will depend entirely on the ratio which sub- 
sists between the quantities of electricity. 

This may be illustrated by an experiment represented in fig- 
ure 19, in which an insulated metallic cylinder communicates 
with the prime conductor of the electrical machine. At the 
end of the cylinder a small pith ball is suspended by a silk thread, 
and its retreating beyond a certain distance is prevented by 
another thread attached to the cylinder. The cylinder is at first 
feebly electrified. The ball is attracted, touches it, and is then 
repelled. The electricity of the cylinder being increased, the 
ball is again attracted ; and thus it is alternately attracted and 
repelled agreeably to our theory. 

To give another example of the same principle, let us consider 
the motions of the little circle of gilt paper, which is attached to 
the needle of the electroscope or of the electrical balance. Let 
us suppose that to this circle, charged with electricity of a cer- 
tain kind, is presented, at some distance nearly parallel to its 
surface, another small circle fixed and electrified, and let this 
second circle for the present be considered a non-conductor, that 
the electricity distributed over its surface, may not be displaced. 

When only the moveable circle is in the balance, the elec- 
tricity will distribute itself over its two faces in the same manner^ 
and in equal proportions, on account of their symmetry. The 
lateral pressures against the exterior air are consequently equal, 



Theory of Electric Attraction and Repulsion. 65 

i 
and no motion can result from them. But, when this electricity 
is subjected to the influence of the fixed circle, it will be attracted 
or repelled, and the pressure exerted against the air will become 
unequal upon the two faces. If it is attracted, its pressure upon 
the air is increased on the side toward the fixed circle ; if it is 
repelled, the reverse takes place. And thus, in the first case, 
the excess of atmospheric pressure will impel the moveable cir- 
cle toward the fixed circle ; and in the second, the motion will 
be in the opposite direction. 

57. We have thus far considered surfaces of such forms that 
the electricity being left to itself, must evidently be distributed 
upon them symmetrically, and produce equal pressures upon the 
opposite parts. In this case the body will evidently remain at 
rest, unless exposed to the action of some foreign force. But 
although it is more difficult to recognise this compensation in 
bodies of less simple forms, it is not less certain that it actually 
takes place in them ; for it is a familiar principle in mechanics 
that the reciprocal actions of the parts of a free system, cannot 
impress upon it any motion of translation, or of rotation about its Mech. 
centre of gravity. 134 < 

This would not be the case if the electric fluid could escape . 
from some part of the body. Take, for example, a needle AA & ' 
of thick wire, either of brass or iron, and let the two ends be 
bent in opposite directions, perpendicular to its length, and let 
them terminate in sharp points. At the centre C make a small 
hole, and adjust to it a conical cap, and place it upon a pivot CM 
so that the needle may turn horizontally. Let the foot of the 
pivot P be screwed to the extremity of the conductor of an elec- 
trical machine. No electricity being excited, the needle will 
remain at rest in its position, but if the machine be put in action, 
the needle will immediately begin to turn, and with increasing * 
rapidity as if it repelled the air by its points. 

To understand this phenomenon distinctly, let us suppose 
that the needle, after being electrified, is covered with a small 
insulating wrapper without weight, and that it is suspended freely 
in a vacuum, by a thread of silk which permits it to turn freely 
about its centre C. In this case, the pressures produced at the 
surface of the electrical stratum, are exerted against the insulat- 
ing wrapper ; but according to the mechanical principle above 
E. &-M. 9 



66 Electricity. 

referred to, they will produce in the system no motion of rotation 
about its centre of gravity, and all the pressures being decom- 
posed in any direction, will mutually destroy each other on the 
opposite sides. Now let us suppose that at a certain part of the 
needle, either the point or any other part, we remove the insu- 
lating wrapper, so that the electricity may escape through this 
aperture ; then the pressure at this part being nothing, the op- 
posite pressure will act without a counterpoise, and cause the 
needle to turn in the direction in which the force is exerted. 

This result could scarcely take place in an absolute vacuum, 
because the electricity of the stratum would be instantly dissi- 
pated when the insulating wrapper was perforated ; but it may 
be obtained in the free air; it is only necessary to sharpen the 
points of the needle to such a degree that the electricity accu- 
mulated there, may overcome the atmospheric pressure. In this 
case, the air serves as a wrapper, and the aperture is made by 
the electricity itself; whereas, in the other case, we supposed it 
to be made artificially. The phenomenon would be precisely 
similar, if the needle, instead of being electrified, were a hollow 
vessel, filled with water or mercury, and its extremities, being 
bent and pointed, were two little canals whose orifices had been 
formed by the pressure of the fluid. The pressure then becom- 
ing nothing at these orifices, that which is exerted on the opposite 
element of the interior surface, would impel the needle, and 
thus cause it to turn in the opposite direction. 

58. In this case, if we take the product of the masses into 
the velocities of all the liquid particles which escape, the pro- 
duct will be constantly equal to the sum of the products of the 
masses into the velocities of the other parts of the needle, and of 
the liquid which turns with it in the opposite direction. The 
same equality must, therefore, obtain in the motion of the elec- 
trified needle ; but the mass of the electrical particles is abso- 
lutely insensible, since the most highly electrified bodies do not 
appear to have their weight increased by a quantity capable of 
being detected by the nicest balances ; it follows, then, that the 
velocity of these particles must be infinitely great ; and no ex- 
ample is, perhaps, better fitted to give us a just idea of this 
velocity. 



Construction of Electrical Machines. 6? 

Before we were made acquainted with the true laws of elec- 
trical equilibrium, it was not known by what means the attrac- 
tion and repulsion, which actually take place between electrical 
particles, could transmit themselves to the material particles of 
bodies ; and this effect was vaguely designated by the word ten- 
sion, which represented the electricity as a spring placed between 
the electrified bodies, and tending to make them approach to, or 
depart from each other. The details into which we have now 
gone, serve to explain how this transmission of force takes place, 
by means of the pressure which the electricity exerts upon the 
surrounding atmosphere, or generally upon the obstacles which 
oppose its dispersion. 



Of the Construction of Electrical Machines, 

59. It has been apparent from our first experiments, that 
to render electrical phenomena conspicuous, it is necessary to 
apply the friction to surfaces of some extent. We accordingly 
make use of a large glass plate or cylinder fitted to turn against 
one or more rubbers, by means of a winch ; and provided 
with an insulated metallic body placed near it, to receive the 
electricity, as it is developed, and to transmit it to other conduct- 
ors, also insulated, as the experiment to be performed may re- 
quire. But, knowing as we now do, that several bodies, thus 
electrified, exert always a mutual action upon each other, we 
have to inquire what is the best arrangement that can be given 
to the several parts of the apparatus ; of what substance ought 
the rubber to be ; what should be the form of the prime conduc- 
tor and the other conductors ; what the form, substance, and di- 
mensions of the insulating supports, in order that they may res- 
pectively answer their purpose in the best manner. These im- 
portant questions we shall answer very briefly. 

There are three principal things to be considered ; namely, 
the plate, the rubber, and the conductors. 

60. Let us first consider the rubber. Whatever may be its 
substance, it is necessary, in order that it may produce an exten- 
sive and continued friction, that it should exactly fit the surface 



68 Electricity, 

of the plate or cylinder, and that it should press it in a great 
number of points. Nothing is better adapted to this purpose 
than cushions stuffc'd with hair, and covered with simple leather, 
which are pressed by a spring against the surface of the glass. 
The leather alone, thus rubbing upon the glass, excites but little 
electricity. We obtain it much more abundantly by covering the 
cushions with a dry amalgam of mercury, zinc, and tin triturated 
together; so that the amalgam is in fact the rubber, and the glass 
the body rubbed.t If we insulate the cushions during the fric- 
tion, and examine the electricity acquired by the glass, we shall 
perceive that it is vitreous ; consequently the cushions take the 
contrary electricity, that is, the resinous, as may be easi-ly 
shown. But in the ordinary use of the machine, we must be 
careful not to insulate the cushions ; on the contrary, they must 
'he made to communicate with the ground by a metallic conduct- 
or ; for we thus obtain the electricity much moro copiously. 

This is always observed in the development of electricity by 
the mutual friction of any two bodies. The excess which each 
of them acquires is always much more sensible when the other 
communicates with the ground than when they are both insulat- 
ed. The circumstance is of great importance, because it seems 
to relate to the manner in which the two electricities are devel- 
oped by friction. But, for the same reason, it is difficult to be 
explained, because our theories apply only to electricity already 
excited, and are as yet but little advanced with respect to elec- 
tricity in its state of disengagement from bodies. We can there- 



t Mr Singer, a late English electrician, who wrote a complete 
treatise on electrical instruments, recommends, as the best amalgam, 
a compound of two parts, by weight, of tin, four of zinc, and seven 
of mercury; the mercury to be heated by itself a little above 100° 
and poured into a wooden box, to which the proper proportions of 
zinc and tin, in a state of fusion are to be added. The box is then 
to be closed, and briskly shaken to unite the ingredients as perfectly 
as possible. When the whole has become cold, it is to be pounded 
in a mortar and reduced to a fine powder; this powder is then mix- 
ed with a portion of hog's lard just sufficient to give it the consisten- 
cy of paste. 



Construction of Electrical Machines, 69 

fore only enunciate the fact as it presents itself in the experi- 
ment, and deduce from it the mechanical conditions to which the 
development of electricity is subject. For this purpose, let us im- 
agine, in the first place, two insulated bodies A and 5, which 
being rubbed, the one against the other, in their natural state, 
acquire, the one a quantity + e of vitreous electricity, the other a 
quantity — e of resinous electricity. I give the negative sign to 
the latter, to indicate that being added to the other, it neutralizes 
it. It is undoubtedly the nature of the two surfaces, and the 
power of the friction which determine this proportion between 
the spaces and the quantities of electricity which attach to each 
of them ; of the nature of the mechanism by which this phenom- 
enon takes place, we are entirely ignorant. But the two elec- 
tricities + e and — e t being once disengaged from their combi- 
nation, there is no doubt that they preserve their individual 
properties, so as to exert their own repulsive force, and mutual- 
ly attract each other. In virtue of their own repulsion, the 
electricity + e, developed upon A, tends to spread itself over 
B, at the points of contact ; and reciprocally, the resinous elec- 
tricity — e, developed upon B, tends to spread itself over A, 
This double tendency is also favoured by the mutual attraction 
which + e anc l — e exert for each other, and in virtue of which 
they endeavour to reunite. Since this diffusion and union do 
not take place, it follows that the unknown power which disen- 
gaged the two electricities 4~ c and — e from each other, and 
separated them, fixing one upon each body, should act also after 
this separation and with sufficient energy to keep them separate 
in spite of the two causes which conspire to make them unite. 
Now it appears that this action of rubbing takes place only at the 
surface in contact, so that it does not prevent either of the two 
electricities 4- e and — e from spreading itself over the surface 
of the body upon which it resides, with the degree of freedom which 
belongs to the greater or less conducting power of this body. For if 
-B, for example, be a conductor, and it be made to communicate 
with the ground by different points of the surface in contact, its 
electricity — e will disappear, and B will return to the natural 
state, without the body A, on that account, losing its excess 4- e\ 
this is constantly seen when we rub an insulated body A against 
a body B not insulated. Now it is very evident that in this state 



70 Electricity. 

of things, the friction developes and maintains upon A a greater 
quantity of electricity than it would do if B were insulated. For, 
in the first case, if A took -f- e, and B — e, in order to retain 
+ e, it would be necessary to overcome, besides its own repul- 
sive force, its attraction to — e ; whereas the latter force does 
not exist when — e has passed off into the ground. For a simi- 
lar reason, if the same body A is successively rubbed against 
two insulated conducting bodies B and B', both of the same na- 
ture, and presenting surfaces of the same kind, but of unequal 
extent, the larger will give a greater quantity of electricity to A\ 
for the disengaged electricity which may fix itself upon B or B', 
being spread over the whole surface of these bodies, it will form 
a thinner stratum, with an equal quantity on the body of the 
larger bulk, and therefore the proper repulsive force of this 
electricity, at the surface in contact, will be less on this body 
than on the other; and hence it follows that the electricity can, 
in this case, be maintained in a greater quantity in a state of 
separation. 

61. Besides these general conditions, the friction of the plate 
of the electrical machine against the insulated cushions, is at- 
tended with a circumstance which renders the effects produc- 
ed much more feeble than when the cushions communicate with 
the ground. It consists in this, that the different parts of the 
plate which present themselves successively in their rotation to 
the rubber have previously passed before the prime conductor, 
where the vitreous electricity which they had acquired is en- 
tirely or almost entirely neutralized ; and thus they are nearly 
in their natural state when they come again between the cush- 
ions.! These different parts, therefore, represent so many insu- 

t The way in which this neutralization takes place, is very evi- 
dent. The parts of the plate which arrive charged with vitreous elec- 
tricity before the prime conductor, decompose by influence its natural 
electricities, repel the vitreous and attract the resinous in the points 
next to the plate. There, this resinous electricity, on account of 
the form of these points, acquiring a great repulsive force, breaks 
through the layer of air which separates it from the plate, and goes 
to neutralize the vitreous electricity adhering to it. The same effect 
would also take place, although less perfectly, if the extremity of the 



Construction of Electrical Machines. 7 I 

lated bodies A, all of the same nature, and in their natural state, 
which are rubbed successively against the same insulated body B. 
Now when the repetition of this friction has developed in B, the 
maximum of electricity — e, which can be maintained upon this 
body in contact with A, notwithstanding the repulsive force 
which this electricity possesses, it is manifest that new friction 
with other bodies A, cannot produce in B any new development 
of electricity. For if a new quantity — e' should be developed 
and should unite itself with — e, the whole repulsive force — e 
— e' would overcome the resistance which opposes its diffusion 
over the surface in contact ; and thus the new quantities of decom- 
posed electricity would be immediately recompounded. Such 
must also be the result of the continued friction of the plate 
of the electrical machine against the cushions when they are in- 
sulated. The parts of the plate which first present themselves 
immediately develope in the cushions all the electricity which 
can be maintained upon them under the influence of the friction ; 
after which the contact of the succeeding parts produces none, 
and the development of the electricity ceases, so that the plate no 
longer offers any thing to be neutralized to the prime conductor, 
whatever number of turns it may make. On the contrary, if the 
cushions communicate with the ground, and are thus constantly 
maintained in their natural state, the parts of the plate as they 
return successively, after being discharged by the prime con- 
ductor, are together with the cushions in the same state as at the 
first contact. They may therefore produce again in the cush- 
ions the decomposition of the natural electricities, become charg- 
ed with a portion of the vitreous necessary to an equilibrium in 
this case, and come again to be neutralized by passing before 
the prime conductor, whence this electricity spreads itself over 
the secondary conductors, upon the surface of which it distrib- 
utes itself according to the laws of electrical equilibrium ; and 
this continual development of electricity ceases only when the 
whole quantity thus spread through the entire system of conduc- 
tors, has acquired such a repulsive force that its action upon the 

prime conductor nearest the plate, instead of being armed with points, 
had only an angular form, so that the escape of the electricity might 
easily take place. 



72 Electricity. 

part of the prime conductor nearest the plate, shall equal the 
opposing action exerted by the electricity, also vitreous, adher- 
ing to the parts of the plate presented to the conductor. It is 
then useless to continue the motion of the machine ; the charge 
of the prime conductor does not increase ; or at most, it only ac- 
quires what is necessary to replace the waste occasioned by the 
air coming in contact with all the electrified surfaces of the plate 
and conductor. 

This minute analysis of the phenomena of the electrical 
machine will suggest to us several important particulars by 
which its construction may be improved. 

62. (1.) It is necessary that the parts of the glass which 
have been successively rubbed, should come before the conductor 
with the least possible loss of the electricity they have acquired. 
For this purpose, we attach to the rubber pieces of oiled silk or 
gummed taffeta, extending over the surface of the glass in the 
direction of the motion. After the glass is electrified, these 
strips adhere to its surface, and preserve it from the contact of 
the air till it has come near to the prime conductor. 

(2.) It is necessary that the prime conductor should have as 
many branches as there are rubbers. We usually employ two 
j*. 2J rubbers F and F', each of which comes in contact with both 
surfaces of the plate. They are placed at the two opposite ex- 
tremities of the same diameter of the plate ; and in order to es- 
tablish with certainty their communication with the ground, the 
back part of each rubber consists of a piece of metal communi- 
cating with the two metallic branches AM, AM', which depart 
from the axis of rotation AA' also metallic. We have then only 
to connect this with the ground ; for this purpose we attach to 
it a chain extending to the floor of the room, or, which is much 
better, communicating by means of a system of conductors with 
a water pipe or well. The prime conductor consists also of two 
branches CB, CB', the parts of which nearest the plate are 
armed with points for the purpose of discharging more easily the 
resinous electricity developed there by the vitreous influence of 
the parts of the plate successively presented to them. But the 
opposite extremities of these branches we never arm with points 
which would rapidly dissipate into the air the electricity acquir- 
ed by the conductor ; on the contrary, they are made to termi- 



Construction of Electrical Machines, 73 

nate in a large ball. Still a conductor thus terminated would be 
saturated with a moderate quantity of electricity. On this ac- ^ 
count it is made to communicate with a system of insulated con- 
ductors, formed of long and narrow cylinders suspended parallel 
to each other. Experiment and theory concur to show, that 
where the lengths and diameters of these cylinders are in proper 
proportion, this arrangement is best adapted to obtain large 
charges with but feeble intensities. It has this advantage 
also, that when we come to turn the plate or cylinder, we can 
cut off the communication between the prime and seconda- 
ry conductors ; for by this means we prevent the dissipation 
of the accumulated electricity which would rapidly escape by 
the points of the prime conductor, when the electricity of the 
plate, by not being renewed, should cease to repel it. 

It is evident that these changes in the communication ought not 
to be made by the direct contact of the hands of the experimen- 
ter, but by means of metallic rods attached to insulating handles. 
When only a momentary communication is required, we usually 
give to these rods the form of two circular arcs AC, A' C, turning p . 23> 
on a hinge about the centre C, and each provided with an in- 
sulating handle M, which ordinarily is a rod of glass covered 
with gum lac. We take one of these rods in the left hand, the 
other in the right ; then opening or closing the angle which they 
form, we can augment or diminish at pleasure the distance AA' 
of the two extremities of the arc, and adapt it to the distance 
between the two conductors which we wish to connect. This 
instrument is called an exciter, because it in fact serves to excite 
sparks between one conductor and another. The instrument 
represented in figure 24 answers the same purpose, although it 
is generally used to discharge jars or batteries, and is hence 
called a discharger. We also employ, as means of communica- 
tion, metallic chains and cords which are suffered to hang from 
one conductor to another, and which are easily removed with 
tubes of glass when we wish to cut off the communication. 

63. After determining the best forms for all the parts of an 
electrical machine, it only remains to say a word respecting in- 
sulation. It is plain that the insulation of the prime and secon- 
dary conductors ought to be as perfect as possible, that they 
may preserve for a long time the electricity which has been 

E. <$r M. 10 



^4 Electricity. 

communicated to them. For this purpose, the supports should 
be as long and thin as consists with convenience and stability. 
Those of the prime conductor are usually glass pillars. They 
should be varnished with gum lac, because this gum insulates 
much better than glass, and is less likely to contract moisture. 
The secondary conducters may be suspended from the ceiling by 
silk cords ; and it would be well, in this case, if the upper part 
of the cords were terminated by a cylinder of gum lac. As to 
other particulars, we proceed according to the principles laid 
down in articles 16 — 24. 

64. We have thus far supposed the rubbers to communicate 
with the ground, and the conductors to be insulated. In this 
case the electricity acquired by the conductors is vitreous. But 
we may also give them the resinous electricity. For this pur- 
pose, we make the branches CB, CB', of the prime conductor 
moveable about the axis CC', and also the two branches AM, 
AM', which connect the rubbers with the ground. If we would 
obtain the resinous electricitv, we turn these branches, as rep- 
resented in figure 25, so that those of the prime conductor, which 
are insulated, shall touch the pieces of metal on the back of 
the rubbers respectively, and those which before communicated 
from the rubbers to the ground are to be placed opposite to the 
rubbed surfaces of the plate. Then the vitreous electricity ac- 
quired by the plate is neutralized in a degree by the resinous 
electricity thus developed by influence in the branches AM, 
AM' ; and, on the contrary, the prime conductor retains all the 
resinous electricity which is developed upon the rubbers. With 
this disposition of the instrument, it is necessary that the points 
with which the branches of the prime conductor are armed, 
should be disposed in such a manner, as to be opposite to, or in 
contact with, the rubbers, in order that their resinous electricity 
may pass into the system of conductors, either immediately and 
by contact, or by influence. Moreover, the supports which sus- 
tain the cushions and which are usually attached to the frame 
work of the machine, ought, in this case, to he of an insulating 
nature, and so arranged as to produce the most perfect insulation. 
It is also important to be able, as we have supposed, to bring be- 
fore the glass plate the two metallic branches AM, AM', which 
communicate with the ground, in order to neutralize all the vit- 



Electroscopes. 75 

reous electricity with which the surface is covered when it comes 
from the rubbers ; for, if it preserved this electricity, it would 
develope none anew when it passed a second time between the 
cushions, and the charge of resinous electricity which the con- 
ductor might acquire, would be much less. 



Of Electroscopes .f 

65. Electroscopes are instruments destined, as their name im- 
ports, to discover the smallest quantities of electricity. We 
have already spoken of that of Coulomb, which is a true elec- 9. 
trical balance suspended by a thread of silk as it comes from 
the silk worm. Other electroscopes are also founded on the 
general principle of the repulsion which takes place between 
bodies charged with similar electricities ; and their greater or 
less sensibility depends on the lightness and facility of motion 
of the substances employed to manifest this repulsion. These 
are usually two long light pieces of straw, or two slips of gold Fig. 26. 
leaf L, Z/, suspended parallel and very near each other, by 
means of two very fine pieces of wire that hook into the rings 
a, a', formed in a common stem or rod, also metallic, which is 
terminated by a knob. By means of this continued communi- 
cation, all the electricity given to the rod T is spread over the 
wires, and thence over the straws or leaves, which immediately 
manifest it by diverging from each other. But since the portion 
communicated is in fact all which is indicated, it must be evi- 
dent that the apparatus will be the more sensible, according as 
these slips are lighter, more free in their motion, and according 
as the rod T, which communicates the electricity, retains a less 
portion of it upon its own surface. For this reason, it is neces- 
sary that the stem should be thin and the knob small, though of 
a size much greater than the stem. To prevent any motion 
from the air, and to screen it from accidental injury, the whole 
apparatus is enclosed in a square glass case, the neck of which 
is covered with gum lac that the insulation may be more perfect. lg * 

f Usually called electrometers in English treatises on Electricity. 



76 Electricity. 

The summit only of the stem appears above the glass, and this 
admits of being turned so that the slips shall diverge parallel to 
one of the faces upon which is traced a small graduated arc, to 
measure the amount of the divergence. It is evident that a 
greater or less divergence will indicate a greater or less degree 
of electricity ; but as the tendency of gravity to bring the slips 
back to a vertical position, augments in proportion as they be- 
come more oblique, it is manifest that the repulsive force which 
supports them is not simply proportional to their divergence, 
but follows a law less simple, depending on the weight of the 
slips and their figure ; and consequently the parts of the graduated 
arc, supposed equal among themselves, do not represent equal 
degrees of electricity. Therefore, when it is proposed to meas- 
ure equal degrees, it is necessary to have recourse to the balance 
of Coulomb or to his electroscope, which alone possesses the 
double advantage of indicating the smallest electrical forces and 
of measuring them at the same time. 

66. We can communicate to electroscopes of whatever descrip- 
tion either the vitreous or resinous electricity, by touching the ex- 
terior knob of the stem with an insulated conductor, charged with 
this kind of electricity. But there is another method equally suited 
to this purpose, which it may be well to explain, since it requires 
only a tube of glass or sealing wax, or other electric, which, on 
being rubbed with a proper substance, produces a known kind 
of electricity, 
Fi 7 Let us suppose, for example, that a stick of sealing wax is 

used, and that the electroscope is that of Coulomb. The circle 
of tinsel C being in contact with the fixed ball A, we rub the 
sealing wax with a cat skin, and present it to the exterior knob 
B of the metallic stem AB at some distance; the needle SC is 
immediately repelled. The repulsion continues as long as the 
sealing wax is presented. If it be brought nearer to the knob, 
the needle is driven to a greater distance ; if it be removed fur- 
ther off, the needle approaches the fixed ball ; if it be entirely 
withdrawn, the needle returns and touches the ball, and remains 
in contact with it at its point of rest. 

All these phenomena are to be referred to the case of influ- 
ence exerted at a distance. The electricity of the stick oLseal- 
ing wax is resinous. It decomposes the combined electricities 



Electroscopes. 77 

of the stem AB and of the fixed ball A ; it attracts the vitreous 
into the exterior knob, and repels the resinous into the fixed ball 
and the circle C of the tinsel which touches it. This circle is 
therefore repelled from the ball, since it is electrified in the same 
way. If the sealing wax is brought nearer, the decomposition 
of the combined electricities increases ; the resinous electricity 
of the fixed ball becomes stronger, and therefore the circle C is 
driven farther off. The contrary takes place if we remove the 
sealing wax. If it is taken away entirely, then the stem and 
the fixed ball are abandoned to their own proper forces, and 
their decomposed electricities again unite ; but they cannot be 
neutralized completely, and the resinous electricity is too feeble 
by whatever the tinsel has taken away. The stem and fixed 
ball, therefore, remain charged with a small excess of vitreous 
electricity, corresponding to the resinous electricity of the tinsel. 
There ought, then, to remain some attraction, and it is only at 
the moment of contact that the union is completed. 

67. This being well understood, nothing is more easy than 
to communicate to the tinsel and to the fixed ball a durable state 
of vitreous electricity. 

For this purpose, touch the exterior knob of the stem 
with the finger, and present at a distance the excited sealing 
wax ; then withdraw the finger, and afterward the sealing wax. 
During the contact, the influence of the sealing wax decomposes 
a portion of the natural electricities of the finger and the stem. 
This influence drives off the resinous electricity into the ground 
on account of the free passage which is afforded by the finger ; 
and it retains the vitreous, which it attracts into the part nearest 
to the stick of sealing wax ; so that if the stem be long enough, 
the tinsel placed at the other end will not be repelled. When 
the finger is withdrawn, this vitreous electricity can no longer 
escape ; and when the sealing wax is withdrawn, it remains free 
upon the surface of the stem and fixed ball ; and then the tinsel 
is repelled. It is necessary to withdraw the finger before the 
stick of sealing wax ; otherwise the excess of vitreous electricity 
would escape into the ground ; or, which amounts to the same 
thing, this excess w r ould be neutralized by resinous electricity 
from the ground, and every thing would return to its natural 
state. 



78 Electricity. 

As a proof that this excess of electricity is really vitreous, 
observe the motions of the tinsel. Since, according to the dis- 
position of the apparatus above supposed, it is not repelled till 
the moment when the sealing wax is withdrawn, it must have the 
same electricity as the fixed ball. Bring the sealing wax again 
toward the exterior knob nearer than before ; it will attract 
toward it the vitreous electricity ; and producing, moreover, a 
decomposition of the natural electricities, it will repel the resin- 
ous into the fixed ball. The circle of tinsel will immediately re- 
turn toward this ball ; and if we do not immediately withdraw 
the sealing wax, it will come into contact. This approach under 
the influence of the sealing wax is the sign by which we may 
recognise all the cases in which the tinsel and the fixed ball 
are charged with vitreous electricity. By proceeding in the 
same way with a tube of glass rubbed with cat skin, or with 
woollen cloth, the tinsel and the fixed ball become charged with 
resinous electricity. 

68. But the same effect may also be produced with sealing 
Fig. 28. wax. For this purpose, take a small glass tube 1 1, at the ex- 
tremity of which attach perpendicularly, by means of soft wax, 
a wire//*, 8 or 9 inches in length. Touch the exterior knob of 
the electroscope with the insulated wire, placing it in such a 
manner that it shall become, as it were, the continuation of the 
stem AB. Then present at some distance the stick of sealing 
wax, and withdraw first the wire and afterward the sealing wax. 
The stem and the fixed ball will be charged with an excess of 
resinous electricity ; for, by the disposition of the several parts 
of the apparatus, the vitreous electricity, which is decomposed, 
is almost entirely attracted into the wire//, nearest the sealing 
wax. Therefore this wire must have an excess of vitreous 
electricity, and thus, by its influence, cause the stem and the 
fixed ball of the electroscope, to possess an excess of resinous 
electricity. 

What we have now remarked may be easily verified by the 
motions of the tinsel. For when we remove the stick of sealing 
wax, it does not return of itself toward the fixed ball as in the 
preceding experiment ; but remains at a distance from it, not- 
withstanding the force of torsion which tends to make it return ; 
and it will withdraw still farther, if we present, at some distance, 



Electroscopes. 79 

the sealing wax to the exterior knob of the electroscope, because 
the influence of the sealing wax augments the quantity of resin- 
ous electricity accumulated in the fixed ball. This repulsion, 
under the influence of the sealing wax, is the sign by which we 
recognise all the cases where the tinsel and the fixed ball are 
both charged with resinous electricity. By proceeding in the 
same way with a glass tube rubbed with woollen, we should 
communicate to the electroscope the vitreous electricity. 

69. We shall now be able to explain why it is necessary to 
give to the wire a length of 8 or 9 inches ; such an extent facil- 
itates the separation of the combined electricities, and the re- 
moval of one or the other with more ease ; for the same reason 
it is useful to give nearly the same length to the metallic stem 
AB of the electroscope. But it is proper always to make it very 
thin, and the knob very small which terminates it, so that small 
quantities of electricity may, on account of the smallness of the 
surface have sufficient force to repel the tinsel of the moveable 
needle, which is one of the most essential properties of the in- 
strument. 

70. The methods which we have given for communicating 

at pleasure the vitreous or resinous electricity, are applicable to 67. 
all kinds of electroscopes. All that we have said with respect to 
the tinsel and the fixed ball, may be said of straws or slips of 
leaf separated by the repulsive force. Here also it is by the 
influence exerted at a distance, that we develope one or the oth- 
er kind of electricity ; and if they are already charged, it is by 
the same signs that we determine the nature of the electricity 
which produces their divergency. But a precaution is requir- 
ed in this case not necessary in the electroscope of Coulomb. 
This is to bring the electrified body toward the knob, gradually 
and at first from a distance, as if we would foresee the nature of 
the electricity. For if the straws or leaves diverge, for exam- 
ple, with vitreous electricity, and we bring toward the stem of 
the electroscope a stick of sealing wax rubbed with woollen, be- 
sides the action of this wax to attract to it the excess of vitreous 
electricity spread over the stem and the straws, a decomposition 
of the combined electricities will also be produced; and the 
electricity of the same name with that of the sealing wax, that 
is, the resinous, will be repelled into the straws. If it should 



80 Electricity. 

happen (o be more than enough to saturate the little vitreoufc 
electricity which still remains in them, they will diverge anew 
but resinously ; and the change from one of these repulsions to 
the other may be so rapid as not to be perceived. It would 
then seem that the original divergence was owing to a resinous 
electricity ; which is a mistake. This will not happen if we 
bring the sealing wax gradually toward the knob, and we 
shall have time to observe the gradual weakening of the first 
repulsion before the developement of the second which succeeds 
it. 

Of the different kinds of electroscopes, that of Coulomb is 
the most easily constructed ; it is also the most sensible, and that 
which best preserves the electricity communicated to it. These 
qualities render it of the greatest utility in all delicate inquir- 
ies, of which I shall soon have occasion to exhibit some striking 
examples. 



Of the Condenser, 

71. Having presented a complete and satisfactory theory of 
the action of electricity, we are prepared to understand the na- 
ture of certain instruments in which it is more powerfully and 
more durably exhibited, either by attracting into a single point 
all the electricity of a system of conductors, by the influence of 
an electricity of a contrary nature, or by employing the perma- 
nent influence of the same quantity of electricity, to produce 
successively the separation of the combined electricities of sev- 
eral conductors presented at a distance. It will only be neces- 
sary to describe these instruments ; their theory will occur of 
itself. 

72. Where a conductor A, insulated and in its natural state, 
is placed in contact with a system of electrified conductors, or 
with a permanent source of electricity, it acquires a determinate 
charge ; but if we bring toward it another body B, in its natural 
state and communicating freely with the ground, the presence of 
this body causes the body A. to receive a stronger charge of 
electricity. In fact, the electricity with which A is at first cov- 



Condenser, 81 

ered, acts upon the combined electricities of £, in proportion as 
that body is brought nearer ; it repels the electricity of the same 
kind into the ground, and attracts that of the opposite kind, which 
fixes itself upon the surface of B nearest to A. But by this same 
attraction, the equilibrium is disturbed in the system of conduc- 
tors with which A communicates. A new quantity of free fluid 
is therefore spread over A, whence results a new decomposition 
of fluid upon £, and so on, till the fluid accumulated upon A is 
brought to a state of equilibrium between the repulsion which it 
exerts upon itself and the attraction of the fluid of B tending to 
retain it. 

All these phenomena, derived directly from the theory, are 
completely confirmed by experiment. 

We communicate to the prime conductor of an electrical 
machine a feeble degree of electricity, after which a metallic 
plate A being taken and held suspended and insulated by its hook Fig. 29. 
C, by means of a glass rod M, this hook is made to touch the 
conductor. The plate thus takes a small quantity of electric- 
ity, which, when it is removed from the conductor, may cause 
a certain degree of divergence in the pith balls of an insulated 
electroscope, formed of two linen threads suspended from a stem 
of copper. 

After this operation, the conductors will have lost so small a 
quantity of electricity, that they may be regarded as having ve- 
ry nearly the same charge as before ; we touch them again in the 
same way, but at the same time holding, below the insulated 
plate A, another plate 22, communicating with the common res- 
ervoir, the ground. The first plate A is then separated from 
the conductors, being still kept under the influence of B ; in this Fig. 30. 
way, it takes a charge of electricity much greater than before, 
as may be ascertained by presenting it anew to the electro- 
scope. It is evident that it is necessary to withdraw A from the 
contact while under the influence of B ; for if B were withdrawn 
first, the fluid accumulated in A would immediately return into 
the system of conductors, according to the laws of its first equi- 
librium. 

If we repeat this experiment, holding at first the plate J9, 
very distant from A, then a little nearer, and finally very near 

E.&M. 11 



82 Electricity. 

to it, we shall find that the charge of A augments more and more. 
This is in fact agreeable to theory ; for the reciprocal attraction 
of B and A ought to augment in proportion as their distance di- 
minishes ; the maximum charge would therefore correspond to 
the case in which the distance of the two plates is absolutely 
nothing. But as we could not come to this limit without ex- 
citing a spark through the air which separates them, we inter- 
pose between them a body which is very thin, and very imper- 
meable to electricity, as a plate of glass, a piece of varnished 
taffeta, or a thin lamina of resin. With this precaution, we may 
diminish the distance of the two plates, almost at pleasure. In- 
struments constructed in this way are called condensers. 

72. The condenser with the glass plate is liable to be cover- 
ed with moisture, which easily adheres to glass and impairs its 
insulating property. The condenser with taffeta cannot be com- 
pared with itself, because the greater or less pressure of the 
plates upon the taffeta, causes the distance to vary, and with it 
the intensity of the condensation. The best method is that in 
which the separation is produced by a simple lamina of resinous 
varnish applied separately to each plate. It is only neces- 
sary to place the plates upon each other without rubbing them ; 
for the friction would develope electricity in the lamina of resin 
which would adhere very strongly to its surface, and which 
might afterward be the cause of error in very delicate experi- 
ments. To render the use of these instruments convenient, we 
give to the plate B a solid foot of metal, and fit to the upper 
surface of A an insulating handle M of varnished glass. The 
whole apparatus is represented- in figure 31. When we would 
make use of it, we place the plates one upon the other ; we touch 
the lower plate B in order to make a communication with the 
ground ; we next touch the electrified bodies with a knob a of 
a wire firmly attached to the upper plate A, which is called the 
collector plate, because it is that in fact which takes the electric- 
ity from the bodies to which it is applied. After the contact, 
we place the foot of the condenser upon a solid table ; then, 
while it is firmly held there, we remove the collector plate by 
the insulating handle M, and test the electricity with which it is 
charged. It is necessary to separate the plates perpendicularly 
to their position ; for if they are separated obliquely, the elec- 



Condenser. 83 

tricky of the collector plate would tend toward the edge of the 
plate nearest to £, and its accumulation there might produce a 
spark that would pierce the lamina of varnish and discharge the 
condenser. It is for this reason that the foot of the instrument 
ought to be kept firmly fixed while we remove the collector 
plate ; for the adhesion of the two plates tends to make them 
slide upon each other obliquely. We must be careful, also, not 
to charge these instruments with a degree of electricity too 
great for the resistance opposed by the double insulating lamina 
which separates the plates ; for if this resistance can be over- 
come, the two accumulated electricities would pierce the laminae 
and unite by an explosion, as they do through the air. This is 
very liable to happen in the condenser with varnished plates, 
and for this reason, it ought to be reserved for very small quan- 
tities of electricity. When the charge is required to be strong, 
it is necessary to make use of the condenser with plates of glass. 
But then, if the plates are not well varnished, the greater part of 
the accumulated electricity is spread over the glass and attached 
to it, so that it does not follow the collector plate when that is re- 
moved. This inconvenience may be remedied by applying to 
the surface of each plate, a disc of thin glass which is fixed there 
and which prevents the electricity from quitting this surface. 
But in order that very strong charges may be preserved in this 
way, it is necessary to prevent lateral discharges by giving to 
the discs a greater diameter than that of the plates, and cover- 
ing the projecting portion of their surface with a thick layer of 
very pure varnish.t 



t A good varnish is very easily obtained by dissolving some seal- 
ing wax in alcohol. For this purpose, it is necessary to pulverize 
it and to let it remain in the alcohol for several days. The opera- 
tion is quickened by warming the alcohol. When we wish to make 
use of this solution, we slightly warm the glass, or the substance to 
which we wish to apply it, and we then put it on with a brush. The 
alcohol is carried off by the action of the air, and the sealing wax 
remains. Over this may be laid a second or third coating, and so 
on. A more perfect insulation is effected by using gum lac in this 
way instead of resin. 



84 Electricity. 

When such a condenser communicates with an electrical 
machine by one of its metallic faces, the other communicat- 
ing with the ground, the latter is in the same state, as if it had 
been brought, without a discharge, very near to a highly charg- 
ed conductor. The union of these circumstances is therefore 
extremely well adapted to produce a strong discharge. Thus 
when we take in one hand the foot of the condenser, which 
makes us partake of its electrical state, and with the other touch 
the collector plate, the accumulated electricities are discharg- 
ed, and unite with much force through the medium of the 
body. This discharge produces a shock in all the organs, 
which is the more violent according as the condenser is larger, 
its charge stronger, and its plates nearer together. This shock 
transmits itself through several persons holding each other by 
the hand, but becomes gradually weaker as it proceeds, and this 
diminution of force is owing doubtless to the resistance which 
the bodies in question, not being perfect conductors, oppose to the 
passage of the electric fluid. 

73. The whole force of condensers may be calculated up- 
on the following principle, which indicates at the same time the 
the manner and the limits of the accumulation which they pro- 
duce. The electricity A being introduced into the collector 
plate, neutralizes at a distance a portion — B, of the contrary 
electricity, upon the lower plate which communicates with the 
ground, and prevents it from escaping. This in its turn fixes, 
in the same way, a portion A' of the electricity of the collector 
plate and takes from it its expansive force. The collector plate 
is therefore in exactly the same situation as if it had only A — A' 
of free electricity ; consequently it must continue to be charged 
until this quantity equals that which it would have taken imme- 
diately from the conductors with which it communicates, if it 
had been placed alone in contact with them, without the influ- 
ence of the lower plate. The ratio of A to — B and of — B to 
A' depends on the greater or less distance between the plates. 
But. in all cases, — B must be weaker than A, independently of 
the sign, so that if A is vitreous and B resinous, these two quan- 
tities united, will become vitreous. For the attractions of the 
particles -f- & upon — B must be less at a distance than it would 
be in contact ; since, therefore, they neutralize — B and take 



Cimdenser. 85 

from it its expansive force through the insulating lamina, they 
must compensate by their number for the weakness of their 
action. Consequently we must always represent B as a frac- 
tion of A. To make myself understood more distinctly, suppose 
B T 9 /o of A, and see what follows from this supposition. 

While + A neutralizes — B through the thickness of the 
insulating lamina, in the same way — B neutralizes a portion A' 
of A ; and the manner of action being exactly the same, the 
proportion neutralized must also be the same, that is, T 9 /o • Thus 
A J will be T 9 /o of £, and as B is itself T %% of A, it follows that A' 
is no X /A of A or T VoVo of A. The excess of A over A', 
which is the portion of electricity that remains free upon the 
collector plate, will therefore be A — tAW °f ^ tnat * s > ^ w ^ 
be x^ff o of A $ a, fraction very nearly equal to j\ of A ; and 
thus this plate will continue to acquire electricity till the fiftieth 
part of its charge equals the quantity which it would naturally 
take from the same conductors, if it were presented to them alone 
and without the influence of the lower plate. Its charge, there- 
fore, under this influence, will be fifty times greater than in the 
state of separation. 

74. The mode of reasoning which we have now made use of, 
shows generally that the condensing force of the instrument de- 
pends on the fraction which expresses the ratio of saturation at 
a distance between its two surfaces. The nearer this fraction 
approaches to unity, the more nearly equal will the quantities of 
electricity be, which may be neutralized through the insulating 
lamina, and the less will be the excess of electricity which re- 
mains free upon the collector plate. The ratio of this excess to 
the whole charge may always be calculated, as in the preceding 
example, and being inverted, it will give the measure of the con- 
densation. 

It is here supposed that we know the value of the fraction which 
expresses the ratio of saturation at a distance between the two 
plates. This we determine by experiment in the following manner; 
we insulate the instrument and charge its collector plate with any 
quantity of electricity, the lower plate communicating with the 
ground. This being done, the communication is broken off; and 
the two plates having become insulated again, they are separat- 
ed parallel to each other with their insulating lamina?, being held 



86 Electricity. 

by their glass handles ; we next apply the trial plane to each of 
them, at a point similarly situated, for example, upon their cir- 
cumference, and measure by the torsion balance, the charges 
thus acquired. They will be proportional to the thickness of 
the electrical strata at the points of contact, and consequently 
to the total quantities of electricity of the two plates, since these 
are supposed equal in magnitude, and the points of contact are 
similarly situated. Thus the charge taken from the collector 
plate may represent A, and the charge taken from the lower 
plate — B ; and the ratio of the latter to the former will be the 
ratio of saturation ; whence we may deduce by calculation the 
measure of the condensing force. This method is more certain 
than to endeavour to determine directly the proportion of conden- 
sation, as it would seem that we might do, by comparing with 
the trial plane the charge which the collector plate receives from 
the same system of conductors when it is alone and when it is 
under the influence of the other plate. For, in order that this 
comparison may be exact, it is necessary that in the two cases, 
the conductors should be charged to exactly the same degree ; 
and of this equality we can never be certain. 

75. The condensing force being determined, the absolute 
effect of the condenser depends still on the absolute quantity 
of electricity which the collector plate would take from the con- 
ductors by which it is charged, if it were placed alone in con- 
tact with them. But, other things being the same, this quantity 
must increase with the surface of the collector plate. Therefore 
condensers of a large diameter will accumulate more elec- 
tricity than those of a smaller diameter, and must give greater 
shocks on being discharged ; and this is in fact confirmed by 
experiment. 

These reciprocal neutralizations which we have made use of 
for the purpose of calculation, may be rendered sensible by the 
following experiment. 

76. After charging a condenser constructed with a plate of 
glass, the lower plate of the condenser communicating with the 
ground, insulate the whole apparatus, and first touch the lower 
plate ; we shall draw from it no electricity ; consequently all 
the electricity upon it is disguised. Then touch the upper 
plate, and a spark will be given ; still the electricity will not all 



Condenser* 87 

be carried off; a considerable portion will remain in a disguised 
state. To render it sensible, touch anew the lower plate. It 
will now give a spark ; for its electricity is not all disguised, 
since we have taken away a part of that which retained it by 
its action at a distance. But by this contact a new portion of 
the latter has become free ; the collector plate will therefore 
give another spark, and so on till the two plates are completely 
discharged. It is easy to determine by calculation the law of 
this progression from the constant ratio of saturation at the dis- 
tance between the two plates. We thus find that the first con- 
tact takes away more electricity than the second ; the second 
more than the third, and so on ; and that these quantities follow 
a decreasing geometrical progression, having for its ratio the ra- 
tio of saturation. 

When we touch both plates at once, all the electricity which 
would have escaped from the two faces by the successive con- 
tacts, is transmitted simultaneously through the body, and this 
single shock completely discharges the condenser. 

77. I have said above that in the condenser with a piece of 
glass and naked plates, the greater part of the accumulated elec- 
tricities does not adhere to the surface of the plates, but attaches 
itself to the opposite faces of the glass. In that case, the two 
plates have properly no other effect than to establish a free 
communication between the different points of each of the two 
faces of this glass, in order that the electricity may easily spread 
itself over them and may also escape, at the moment of the dis- 
charge, from all their points at once. This may be easily veri- 
fied by experiment ; for this purpose, after having charged such 
a condenser, place it upon an insulator ; then with the hand re- 
move the upper plate by its insulating handle, and touch it ; we 
shall receive from it only a small spark, and the expansive force 
will remain with the other plate. This being done, remove also 
the glass plate, lifting it by one of its edges, and touch the low- 
er plate ; this will give a spark in its turn, but also very small. 
It follows from this that the accumulated electricities have re- 
mained attached to the two faces of the glass plate ; and in fact 
if we replace it between the two insulated plates of the conden- 
ser, without communicating to them, or to it, any new electricity, 
the condenser will be found to be recharged of itself almost as 



88 Electricity. 

strongly as at first. Or otherwise, without replacing the glass 
between the two plates, if we apply both hands directly to its two 
faces, so as to touch a great number of points at once, we shall 
feel a discharge, just as if the glass had again been covered with 
the plate ; because the extent of contact of the hands permits 
a large number of points of the two surfaces to discharge them- 
selves at once. But if, instead of touching the faces of the glass 
with the open hands, we merely move over them the extremities 
of the fingers, we shall only perceive a slight sparkling and a 
local discharge in the points touched ; no general discharge, 
however, will take place, and thus we shall be exposed to no 
violent shocks. 

78. iEpinus, who was indeed the real inventor of this instru- 
ment, contrived an experiment in some respects the reverse of 
the preceding, which shows very evidently what is the precise 
use of the insulating lamina interposed between the two plates. 
He employed for plates two large circular pieces of wood cover- 
ed with sheets of tin ; and having brought them toward each 
other in a parallel direction, without any thing being interposed 
except the stratum of air which separated them, he caused the 
upper plate to communicate with the conductors of an electrical 
machine, the lower communicating with the ground. This ap- 
paratus, it will be perceived, is a true condenser, an aerial lam- 
ina taking the place of the varnish ; it is charged, also, in the 
same way as a condenser is charged, and it gives a shock when, 
the lower plate being touched with one hand, the upper is touched 
with the other. In order to obtain considerable shocks from this 
apparatus, it is necessary to employ large plates ; for since we are 
obliged to keep them at a considerable distance that sparks 
may not escape from them directly through the air, the extent of 
surface must compensate for the weakness of the condensing force. 
Besides, this extent seems to be one cause which retards the 
spark when the plates approach parallel to one another. Its 
effect is in a degree the reverse of the effect of points. The 
only difference between this and the common condenser is, that 
the surfaces of the insulating lamina have no real existence, ex- 
cept when the two plates are in presence of each other, for they 
are nothing else but the aerial limits of the surfaces which the 
two plates mutually present to each other. 



Condenser. 89 

79. Although iEpinus actually invented the condenser, as we 
have said, and gave its true theory, as may be seen in his trea- 
tise, it was Volta, who by uniting it to the electroscope, render- 
ed it useful in discovering and making sensible the most feeble 
sources of electricity. 

Indeed, we often meet, in physical inquiries, with sources of 
electricity capable of affording only very feeble repulsive forces, 
and which fail entirely when they have attained a certain limit; 
but which, if we destroy the electricity thus produced, develope 
it anew. Of this we shall soon present several examples. Sup- 
pose a communication between one of these constant sources of 
electricity and the collector plate of the condenser whose insu- 
lating lamina is exceedingly thin, a single layer of varnish, for 
example. It is evident that the electricity from this source will 
go on accumulating in the condenser till the quantity not dis- 
guised is equal to what the collector plate would receive directly 
from the same source. Let us denote this quantity by E. When 
we have reached the limit in question, if we separate the con- 
denser from the source of electricity, and remove the collector 
plate, its charge will be equal to the quantity E multiplied by 
the condensing force. It may therefore become sensible, how- 
ever weak E may be, if the ratio of saturation differ little from 
unity, that is, if the distance between the plates of the condenser 
is very small, a condition which the layer of varnish perfectly 
fulfils. 

In order to unite the indications of this instrument with those 
of the straw electroscope, which Volta commonly used as being 
the most portable and the most convenient, we unscrew the up- 
per knob from the stem, and substitute in the place of it, the 
lower plate of the condenser. This plate is then insulated by 
the glass case of the electroscope. It is made to communicate 
directly by a metallic wire with the constant source of electric- 
ity, and we merely touch the upper plate to make it communi- 
cate with the ground. With this arrangement, it is the lower 
plate which collects the electricity. When we think the charge 
sufficient, we separate it from the constant source without touch- 
ing it, keeping for that purpose an insulating rod ; we then re- 
move the upper plate by its insulating handle. The electricity 
of the lower plate, becoming free, manifests its repulsive force by 
E.S/M. 12 



90 Electricity. 

the divergence of the straws. It is then easy to determine its 
nature by the usual tests. It is sometimes more convenient to 
make the constant source communicate with the upper plate of 
the condenser ; we then touch that which communicates with the 
straws. When the instrument is charged, we cease to touch it ; 
it is separated from the source of the electricity, and the upper 
plate is removed which carries away with it the electricity which 
it had acquired. Then the lower plate which is left insulated, 
preserves the contrary electricity and manifests it by the diver- 
gence of the straws. Its charge is, in this way, somewhat less 
than that of the collector plate, in the first method, since the 
ratio of saturation at a distance is always fractional. But the 
difference will not be sensible, if, as we suppose, the lamina is 
very thin, because this ratio will then approach exceedingly 
near to unity. It is only necessary to remember that this elec- 
tricity is of a different nature from that of the source. 

It is evident that we might equally well apply the condenser 
to the electroscope of Coulomb ; but as the method is exactly 
the same, it is unnecessary to describe it here. 



Of the Elect rophorus. 

80. When a body is electrified and insulated, if we bring 
toward it another body not insulated, the latter will take the 
contrary electricity, and if it be suddenly insulated, it will be 
free to be charged with this electricity. This has been shown 
several times in the preceding sections, and may be proved again 
in different ways. 

We charge the conductors of the machine with a certain 
quantity of electricity, and bring toward them at a distance, a 
metallic disc supported by a glass rod. If we withdraw this 
disc without having touched it, it will be found to be in its natu- 
ral state ; but if we touch it while within the influence of the 
conductors, and then remove it, first taking off the hand, we 
shall find it charged with electricity the opposite to that of the 
conductors. 

We take a metallic disc supported upon a stand, insulate it 
and give it a spark ; after which we use it as in the preceding 



Electrophorus. 91 

experiment, to charge another metallic disc, by touching it and 
then insulating it. This phenomenon is renewed until the elec- 
tricity of the insulated disc has been entirely lost by the contact 
of the air. 

81. In order to know what takes place with respect to the 
electricity of this disc, while it is thus acting by influence, we 
have only to make the lower surface of the disc communicate 
with an electroscope consisting of threads, insulated like the 
disc; the threads instantly diverge. But as the uninsulated 
disc approaches, their divergence diminishes; it finally becomes 
to appearance nothing, and the electricity seems to be destroyed. 
But it is in fact only disguised ; for when the disc which commu- 
nicates with the ground is withdrawn, the threads begin to di- 
verge anew as strongly as at first. 

The decomposition of the natural electricities of the presented 
body, and consequently the quantity of electricity with which it 
becomes charged, augments according as its distance from the 
electrified body diminishes, and it would be at the highest de- 
gree of intensity if this distance were nothing. But we can- 
not diminish it indefinitely without exciting a spark between the 
two bodies. It is for this reason that we interpose between 
them a thin plate formed of some substance impervious to elec- 
tricity, as a plate of glass or a layer of resin. 

In order to show the application of this method, we insulate 
a metallic disc, the lower plate of a condenser, for instance ; we 
protect it with a plate of glass and give it a spark. Upon this 
plate we place the other plate of the condenser which is provid- 
ed with an insulating handle ; we touch its upper surface for an 
instant ; we afterward remove it by its handle and find it charg- 
ed with electricity the opposite to that of the insulated disc. 
This experiment may be repeated as many times as we/please; 
and for this reason the instrument has received the name of 
electrophorus , that is, a bearer of electricity. 

82. We perceive that the condenser and the electrophorus 
are both founded upon the electrical action exerted at a distance. 
But in the condenser, we make use of the presence of another 
body communicating with the ground to augment the charge of 
an insulated body, while in the electrophorus it is the insulat- 
ed and electrified body by which the accumulation is produced. 



Fig. 33 



92 Electricity, 

An electropborus may be constructed in which the thickness 
of the insulating lamina shall be altogether insensible. For this 
purpose, we have only to employ for the lower disc a plate of 
glass or a layer of resin electrified by friction. These substan- 
ces strongly retaining the electricity, we place the upper disc 
immediately upon the surface, without their imparting to it any 
considerable quantity of the fluid ; while the influence, exerted 
in decomposing the natural electricities of this disc, will be very 
great. The most common electrophorus is constructed in this 
way with a cake of resin run into a metallic dish. We electri- 
fy the surface of this cake by rubbing it with a dry cat skin. It 
takes the resinous electricity, and its influence causes in the 
upper plate the vitreous electricity. This apparatus is of use 
in chemical inquiries in which we have frequent occasion for 
electricity. 

83. When the apparatus is charged and placed upon the 
resin, the vitreous electricity which resides upon its lower sur- 
face, and the contrary electricity developed upon the resin, mu- 
tually neutralize each other, and neither has a tendency to es- 
cape. Consequently, they cannot be dissipated by the contact 
of the air, which could hardly insinuate itself between the sur- 
faces where they reside. An instrument thus charged ought to 
preserve for a long time its two electricities, and they are found 
indeed to continue whole months if the electrophorus is kept in 
a dry place. 

Nevertheless the permanent attraction of the two opposite 
electricities must gradually overcome the resistance w r hich the 
resin opposes to the disengagement of its own resinous electricity, 
and to the introduction of the vitreous electricity of the plate. 
This is probably the only cause why, after a longer or shorter 
time, the electrophorus is finally found to be discharged, and 
its different parts reduced to their natural state. 

The effects of this reciprocal attraction may be accelerated by 
greatly increasing its energy. For this purpose, when the electro- 
phorus is charged, remove the metallic plate and place it anew upon 
the resin, not parallel to its plane and in the direction of its sur- 
face, but obliquely and with the circumference toward the resin. 
Then its vitreous electricity accumulating almost entirely in the 
part of its circumference which touches the resin, will take a 



Electrophorus. 93 

much greater repulsive force. It will leave the plate, complete- 
ly neutralize the points toward which it is directed, and after 
several contacts in different parts, the cake of resin, will be found 
to be entirely discharged. 

84. We hence derive a curious experiment. Instead of re* 
storing to the resin the vitreous electricity developed by its influ- 
ence in the metallic plate, apply it to another cake of resin 
which is in its natural state ; it will, in like manner, attach itself 
to the surface of this plate, which will thus be electrified vitre- 
ously, and will thus become capable in its turn of developing by 
its influence the resinous electricity. When the second cake 
has been charged in this way, place a metallic plate upon its sur- 
face ; we shall have an electrophorus affording an electricity the 
opposite to the first. We can make use of this in the same way 
to charge the surface of a thin cake with resinous electricity ; 
and this series may be extended to any number of cakes which 
will be electrified alternately with vitreous and resinous elec- 
tricity. 

85. By this process we can electrify also the surface of each 
cake only in certain determinate parts. For this purpose, it is 
sufficient to adapt to the disc which conveys the electricity a 
stem and a metallic knob like those of the collector plate of the 
condenser. Then if we touch the resin with this knob, the elec- 
tricity will flow entirely to the point of contact. By taking a suc- 
cession of points, we can trace the outline of a proposed figure. 

If we would render these points visible, we have only to 
sprinkle over the surface of the resin some light non-conducting 
powder, as the dust of resin or sulphur. The small particles of 
dust attach themselves only to the electrified parts, so that by 
inverting the cake, all those not thus retained fall off by their 
own weight, and the electrified lines remain covered with these 
particles. We observe that the particles of dust take regular 
but different arrangements according to the nature of the elec- 
tricity by which they are retained ; and hence by forming lines 
with the two electricities upon different parts of the same cake, 
we obtain at the same time two sorts of figures. This curious 
experiment was first performed by Lichtenberg, a German 
philosopher, and the figures thus traced are called Lichtenberg's 
figures. 



94 Electricity. 

To render this phenomenon more apparent, we make use of 
a mixture of sulphur and red lead rubbed together in a mortar. 
The friction thus produced electrifies the sulphur vitreously and 
the red lead resinously. We put this powder into a kind of bel- 
lows which serves to throw it over the cake of electrified resin. 
Then the two substances, attaching to the cake, become separate 
and distinct both by their arrangement and their colour ; the 
sulphur being yellow and the lead red. 

Soon after this discovery, some German philosophers remark- 
ed that the powder of resin, thus spread over an electrified cake, 
exhibited very slight progressive motions, which appeared how- 
ever not to have any regularity. Upon this, a theory was soon 
formed ; but more attentive observers discovered that these mo- 
tions were produced by a little insect, called acarus^ which is 
often found in the powder of resin. 



Of the Ley den Jar. 

86. In the preceding articles, we have examined the phenom- 
ena which are produced by the vitreous and resinous electrici- 
ties, when disguised by each other in virtue of their action at a 
distance. We have seen that when they are in this state, if we 
present to them conducting bodies which communicate from 
one to the other, they dart with force upon these conductors, 
unite, and thus return to their natural state of combination. 

The experiments which we are about to perform relate to 
the same kind of action, and are to be explained on precisely 
the same principles ; but they are worthy of particular attention 
because they furnish powerful means of accumulating the elec- 
tric force, and because they give rise to numerous phenomena 
which require this accumulation. 

We take a glass vessel, as a tumbler, for example, partly 
filled with water, and holding it in the hand, we introduce 
into the water a wire or other conductor communicating with the 
prime conductor of an electrical machine. After a few turns of 
the plate or cylinder, if we attempt to remove the conductor with 
one hand, holding the vessel always in the other, we shall receive 



Leyden Jar. 95 

a shock which will be the more violent according as the vessel 
is larger, the machine more powerful, and continued in action 
for a longer time. 

87. This experiment, which was performed long before the 
invention of the condenser and the electrophorus, and before 
electricity was reduced to a theory, was the result of accident, 
but of an accident that excited attention. It first presented itself 
at Leyden to Cuneus and Muschenbroeck. The phenomenon was 
to them an occasion of surprise and even of terror. It was re- 
peated every where, and being soon familiarized with the 
particulars which had at first excited so much apprehension 
philosophers attempted to discover the arrangement best fitted 
to produce an effect so wonderful. They first discovered the 
necessity of a conducting substance, as water, mercury, or sheets 
of metal applied to the inner surface of the vessel ; they soon 
perceived also the importance of an exterior coating of a con- 
ducting nature, as the hand performed this office in a very im- 
perfect manner. Finally they discovered that it was indispen- 
sable to cut off all communication between the inside and outside 
of the vessel, or rather between the inside and outside coatings, 
except at the instant of the explosion. 

These conditions are fulfilled in the best manner by taking a 
phial or jar of common flint glass, and pasting or glueing upon 
the outside a thin sheet of metal, as tin foil, the inside being 
coated in the same manner, or filled with leaves of metal. A 
metallic rod terminated without by a ball, passes through the 
stopper of the jar and serves to convey the electricity to the in- 
terior. The stopper and a part of the neck are usually varnish- 
ed on the outside. This instrument, which is represented in fig- 
ure 34, is generally called the Leyden jar, from the name of the 
city where its properties were first observed. 

88. The theory of the instrument agrees so exactly with that 
of the condenser, that almost the same language may be used 
with respect to both. 

The electricity which is introduced within the jar, and 
which we will suppose to be of the vitreous kind, decom- 
poses by its influence the natural electricities of the outer sur- 
face, drives off the vitreous, fixes the resinous, and by the 
reciprocal attraction of the resinous is itself partly fixed in 



96 Electricity. 

turn ; and thus the jar forms a true condenser. When a conv 
munication is made by the hand or by both hands between its 
two faces, the two electricities accumulated there rush toward 
each other with great rapidity, and traversing the bodily organs 
produce in them a violent shock ; or, which is the same thing, 
the body which is the medium of communication suffers a rapid 
decomposition of its natural electricities, each of which tends to 
that surface of the jar where the opposite electricity resides. 
This explanation may be verified in every particular by 
experiments similar to those employed in the case of the conden- 
ser. Generally, the Leyden jar is simply a condenser, in which 
the insulating layer is curved, and which has for its coating or 
armour, as it is sometimes called, on the outside, the sheet of 
metal with which the jar is covered, and within, the conducting 
substance with which the jar is filled or covered. 

89. When an electrified Leyden jar is suspended in the air, 
the absorbing action of that fluid can act only on the portion of 
electricity which is free upon either surface of the glass, and 
the reciprocal action of the two disguised electricities serves to 
protect them both. This is very evident from the long time 
which Leyden jars of thin glass take to discharge themselves 
completely, when they are insulated and when the direct com- 
munication of their two surfaces is interrupted by a layer of 
pure gum lac. 

If we examine, at different times, the progress of this absorp- 
tion, by touching the two surfaces with the trial plane, we shall 
find that there have been developed upon each quantities of free 
electricity, of a contrary nature, which finally become sensibly 
equal; after which they maintain themselves in this state of 
equality until both are completely exhausted. We are able, by 
means of the calculus, to account very exactly for this phenom- 
enon, according to the laws of the absorption of electricity by 
the air. When, however, the equality of the charges is thus 
established upon the two surfaces, if we spread upon each a non- 
conducting powder, it would evidently adhere by the attraction 
of the free electricity ; and if, moreover, the electricity were not 
strong enough to repel the particles, they would thus be pre- 
served from the contact of the air ; and thus, there being no 
waste, the jar will remain charged for an indefinite time. This 



Electric Battery. 97 

we in fact observe, when the two surfaces of a thin glass jar, 
after being charged, are covered with a mixture of sulphur and 
red lead, of which we have spoken above. If we suspend such a 
jar by a cord along a dry wall, it will preserve its electricity 
for months. 

90. When we are employed in electrical experiments, we 
ought never to lose sight of the influence derived from the con- 
tact of the air. Overlooking this, we are apt to believe, for in- 
stance, that a Leyden jar, or other instrument of the kind, may 
be charged merely by receiving the electricity of the machine 
upon one of its faces, without communicating by the other with 
the ground ; for, indeed, a jar thus insulated is gradually charged 
especially if it is electrified for a long time. But this is because 
the electricity of its other surface, repelled and rendered free 
by influence at a distance, is exposed to the absorbing action of 
the air which slowly diminishes it, and thus permits the accumu- 
lation of a certain quantity of electricity upon the surface com- 
municating directly with the machine. To make this effect con- 
spicuous, we have only to arm the outer surface with several 
points ; the jar, although insulated in the air, is charged almost 
as strongly as if the surface armed with points had communicated 
directly with the ground. 



Of the Electric Battery. 

91. When we wish to accumulate a large quantity of elec- 
tricity, we form several Leyden jars of a large size, coating the 
two surfaces with tin foil, and connecting the interior surfa- 
ces together, and the exterior together, so that when they are 
charged by communicating with the conductor of an electrical 
machine, they may all be discharged at the same time. This ap- 
paratus is called an electric battery ; it is represented in figure 
35. It is usually placed upon an insulating support, which com- 
municates with a metallic conductor that may be removed and 
replaced at pleasure. 

The greater the extent of armed surface a battery contains, 
the more electricity it accumulates, the action of the machine 
being the same ; it requires also more time to charge it. Gen- 
E. &M. 13 



98 Electricity, 

erally, when we make use of large batteries, it is useful to sepa- 
rate them into several parcels in order to be able to proportion 
the quantity of electricity to the effects to be produced. By 
this means we are able also to charge batteries more rapidly 
with the same machine. 

92. Suppose any number of Leyden jars, or armed surfaces 
of glass, suspended under each other by metallic conductors, as 
represented in figure 36. We attach the first to a cord of silk 
£, and make the last communicate with the ground. We then con- 
vey upon the upper face A l , the electricity of the machine 
which we suppose vitreous ; it is evident that all the lower plates 
will be charged at the same time with the first, by the successive 
repulsions of the electricity of one into the other. But both 
reasoning and experiment show, that in this way of charging by 
cascade, as it is called, the decomposition of the natural electric- 
ities is weakened very fast, as we recede from the prime con- 
ductor; so that if we take only a small number of plates, the 
last are scarcely charged at all. Moreover, if we make the 
first and last links of the chain communicate with each other by 
their opposite faces, we obtain the discharge of the quantities of 
electricity only which they have individually acquired ; and 
those of the intermediate plates recombine of themselves without 
producing any effect; whereas we may avail ourselves of their 
power also, if, after having charged the system by cascade, we 
separate its successive parts in order to make the faces charged 
with the same electricity communicate with each other, and 
then discharge them simultaneously. The same method may be 
advantageously employed in charging large batteries. For this 
end, it is necessary to separate them into several parcels, and to 
place them upon insulating feet, as represented in figure 37. If 
we wish to charge them all or only a part of them, we at first es- 
tablish a communication between the successive faces B 1 ,A 2 ,B 21 
^ 3 , .... by means of the metallic rods C 15 C 2 , . . . . which 
pass through rings provided for this purpose ; and we make the 
last face B n communicate with the ground. Afterward, when the 
charge is supposed to be sufficient, we destroy the communica- 
tion of the face B n with the ground. We may then safely re- 
move, one after the other, the metal rods C n C 2 . . ; for when 
we remove C n for instance, no discharge can take place, for 



. Electric Battery. 99 

the electricity B l is entirely retained by ^,,and the electricity 
A 2 almost entirely by B 2 . Nevertheless, we shall thus receive 
a feeble spark arising from the excess of A 2 over B 2 . This 
being done, and the partial batteries being thus separated, we 
establish communications between their surfaces A A ,A 2 , ... by 
throwing on the same metal rods C n C 2 , . . . (if we lay them 
on, we should be exposed to a discharge ;) these rods meeting 
the conductors by which the parts of each battery are connect- 
ed, naturally place them in communication. Each time the rod 
falls upon two consecutive parts, it excites a spark between them 
which comes from the inequality of the charges acquired during 
the first arrangement. When the batteries are all united, we 
can discharge them all at a single contact, by making the com- 
munication between the extreme faces A l and B n ; or, if we 
please, we can first charge them completely by a renewed mo- 
tion of the machine. 

In these operations, it is important to have an electrometer, 
or, as it is sometimes called, a regulator to point out at each in- 
stant the state of the battery. For, at a certain point of intensi- 
ty, the portion of electricity of the faces A may have a repulsive 
force sufficient to overcome the resistance of the air, and by 
rushing with an explosion toward a face 5, the battery would be 
discharged, and some of the jars perhaps broken, because all the 
force of the shock tends then toward a single point of the coating. 
To avoid an accident of this kind, we screw upon the conductors 
communicating with the faces A, a small pendulum having a me- 
tallic rod TT, and a small rod of ivory carrying upon its extrem- 
ity a small ball b of elder pith. The free fluid of the faces A, F ' l & 38 - 
exerting its repulsive force upon this pendulum, repels it from the 
stem ; and its divergences are measured by a graduated arc 
traced upon the semicircle c c. It is evident that this instru- 
ment gives no absolute measure of the electricity accumulated ; 
but affords at least a constant indication by which we can be 
guided, when we have determined by experiment, once for all, 
the degive of repulsion at which a spontaneous discharge is to 
be apprehended. 

In discharging batteries, we make use of the exciter or dis- 
charger already described. We connect one extremity or knob 
with a face A, and the other with a face £, and the discharge 



100 Electricity. 

takes place through this conductor. When we have occasion 
to use large batteries, care should be taken how we expose our- 
selves, by becoming a part of the circuit ; for a discharge through 
the body might be attended with serious consequences. 



Of the Electric Pile and of the Phenomena presented by Crystals 
capable of being electrified by Heat. 

93. While on the subject of charging by cascade, I shall 
present some results which will be found useful hereafter when 
we come to treat of galvanism and magnetism. They will also 
afford some new examples of the action of disguised electricity. 
Fig. 39. Imagine a series of glass plates, having the two surfaces 
coated with metal, and arranged parallel to each other in such 
a way that the face B x of the first shall communicate by a 
wire with the face A 2 of the second ; the face B 2 of the second 
with the face A 3 of the third ; and so on to the last, the lower 
face B n of this last communicating with the ground. Let us sup- 
pose that, the whole apparatus being insulated, we make the first 
face A x communicate with the prime conductor of a powerful ma- 
chine, and that after having thus electrified it by cascade for some 
time, we interrupt the communication with the conductor and 
with the ground by means of non-conducting rods. It is propos- 
ed to find what will be, after a certain interval, the electrical 
state of the different parts of the apparatus. 

To determine this, it is necessary to consider that at the mo- 
ment when the communication is broken, the first face^ contains 
a certain electrical charge, in part free, and in part disguised by 
the electricity of a contrary nature which it has itself attracted 
and fixed upon the second face B x ; it is the same with the faces 
A 2 and J3 2 , with A 3 and 2? 3 , and so on through all the others. 
Of all these quantities there is only the charge A x which is for- 
eign to the apparatus ; all the others being derived from the 
simple decomposition of the natural electricities. The absolute 
intensity of decomposition varies from one plate to another ; but 
all which is excited upon each is not sensible ; there is nothing 
sensible except the portions of free electricity, which are all of 
the same nature with that belonging to A x . 



Electric Piles. 101 

Now if the apparatus in this state were exposed in a perfect- 
ly non-conducting medium, it is evident that this state of equilib- 
rium would continue without change ; but if it were surrounded 
by an absorbing medium, as the air, it would gradually lose its 
electricity. To understand how this would take place, we must 
remember that in the same state of the air, and for a surface of 
the same form, the waste is proportional to the whole quantity 
of free electricity which resides upon it. Thus, in the first in- 
stants, the loss will be greater for the first face A , than for the 
second A a , because the latter has less free electricity; so also it 
will be greater for A 2 than for A^ and so on to the last face B n , 
where it will be nothing, because upon this face there is no free 
electricity. But by this series of unequal losses, free electricity 
will be developed. For the equilibrium before established did 
not exist between the portions of free electricity of the different 
faces, but between their absolute charges ; and since the first 
charge A l is weakened, it can no longer neutralize upon B l all 
which it neutralized before ; it is the same with respect to the 
action of A 2 upon 2? 2 , and so on to the face B n . The electric- 
ity of this face being no longer completely neutralized, a por- 
tion becomes free, and this portion, at first very small, grad- 
ually augments. For although, from the instant that it first ap- 
pears, it is continually exposed to the absorbing action of the 
air, yet from its weakness, it loses at first less than the free por- 
tions of the other faces ; hence the change of equilibrium goes 
on gradually in the same way, the loss of free electricity dimin- 
ishing more and more upon the first face and increasing upon 
the last, and upon the intermediate faces, varying between these 
two extremes. No limit can be assigned, therefore, to these 
variations, except it be the equality of the quantities of free 
electricity residing upon the two extreme faces of the apparatus, 
which will also reduce their charges to an equality. Then the 
disposition of the electricity will generally be symmetrical, as 
we proceed from these two faces toward the centre of the pile ; 
the quantities of free electricity will be of a contrary nature on 
each side of this centre, gradually decreasing as we approach 
it ; and at the centre they will be nothing, and we may touch 
the plate which is placed there without experiencing any shock. 
But if we break the pile at this place, or at any other, and insulate 



102 Electricity. 

the parts, there will gradually be developed at the broken ex- 
tremitj r , a certain quantity of free electricity, which will be of 
a contrary nature to that of the other extremity which was left 
untouched. 

This result is agreeable to theory, and, as I have satisfied 
myself, is perfectly confirmed by actual experiment. 

The phenomena which are presented by minerals capable 
of being electrified by heat, are analogous to those we have 
described ; and we can scarcely doubt that nature has provided 
them with a similar apparatus, that is, with an electric pile com- 
posed of an infinite number of parallel plates. The mere detail 
of the facts will be sufficient to establish this truth. 

I shall take as an example the variety of the tourmaline 
denominated by M. Hauy isogone ; it has the form of a prism 
with nine faces, terminated at one end by a summit of three 
faces, and at the other by a summit of six faces. When this 
stone is exposed to a temperature less than 98° of Fahrenheit, 
it offers no signs of electricity ; but if v/e immerse it for some 
minutes in boiling water, and then, holding it with a pair of small 
pincers applied to the middle of the prism, we present it to the 
disc of an electroscope or to the small pendulum, already charged 
with a known electricity, we shall see that it is attracted by one 
end and repelled by the other. The summit with three faces 
possesses the resinous electricity, and the summit with six faces 
the vitreous. By making the electroscope very sensible, we 
find that each kind of electricity goes on decreasing rapidly 
from the summit where it resides ; that it becomes very feeble 
at a small distance from each extremity of the prism ; and that 
from this point to the centre, the mineral appears to be in its 
natural state ; in a word, the effects are absolutely the same as 
in the insulated electric pile described above. 

Many other crystals have since been found to exhibit sim- 
ilar phenomena. Several are more sensible in this way than 
the tourmaline, a small increase of heat being sufficient to elec- 
trify them. M. Hauy, who has made many curious researches 
on this subject, has remarked that the property in question be- 
longs only to crystals whose forms are not symmetrical, and 
that the parts where the opposite electric poles reside, vary al- 
ways from symmetry, as the two extremities of the prism of the 
tourmaline. 



Mechanical and Chemical Effects of Electricity, 103 

It is possible that a very great depression of temperature in the 
case of the tourmaline might destroy its electrical equilibrium, as 
an elevation of temperature is known to do, or that it might be 
destroyed by a less degree of heat, if the stone were previously 
exposed to extreme cold. These particulars, which might serve 
to clear up the mystery of the electrification of this mineral, 
deserve to be examined. 

When melted sulphur is poured into an iron basin, and suf- 
fered to cool in this basin while insulated, we find that it ac- 
quires the resinous electricity, and the iron the vitreous. This 
fact seems to indicate what takes place in each element of the 
tourmaline and of the other crystals which are electrified by 
heat. A series of such elements, being placed in contact with 
each other, would probably form a true electric pile, in which 
the insulation and separation of the plates would be effected by 
the nonconductibility of the substance of the crystal. 



Mechanical and Chemical Effects produced by the Repulsive Force of 
accumulated Electricities, 



94. We have already remarked more than once, that the 
electricity spread over the surface of conducting bodies, exerts 
a contrary pressure upon the atmosphere which retains it at this 
surface by its weight. We have seen that this reaction, which 
is always proportional to the square of the thickness of the 
electric stratum, may become sufficiently powerful to overcome 
the resistance opposed by the air. Then the electricity escapes 
through the particles of the air. Hence we infer, that at higher 
degrees of accumulation, the electricity becomes capable of 
breaking through substances much more dense than the air, and 
even of separating their particles. This is confirmed by exper- 
iment. 

The force of an electric battery, when highly charged, is 
sufficient to break cylinders of wood through which it is made to 
pass. It inflames certain combustible bodies, as phosphorus, 
ether, and other spirits, that is, it causes them to combine with 
the oxygen of the air, especially if they have been previously 



104 Electricity. 

warmed. It destroys life when it is made to pass through the 
body of an animal, and the flesh soon putrefies like that of ani- 
mals killed by lightning. It passes also through plates of glass 
lengthwise and breaks them, provided their surfaces are pol- 
ished ; for otherwise the glass would be a conductor and the 
discharge might pass without breaking it. If transmitted along 
a fine wire of iron, silver, or copper, it melts it into little globules. 
With a degree of accumulation still more intense, these wires 
and even thin leaves of metal are suddenly volatilized. 

It is evident that such a force might, by a similar action, 
produce in liquid or gaseous substances, all the phenomena 
which result naturally from a strong compression or from a sud- 
den elevation of temperature ; and this is in fact observed to take 
place. Thus the electric discharge, even that of a simple Ley- 
den jar, inflames hydrogen and oxygen when they are mix^ed 
together in the proportion of about two parts by bulk of hydrogen 
to one of oxygen ; and the residuum is water, or rather the vapour 
of water, elevated to a high temperature by the great quantity of 
caloric which the combination disengages. The most convenient 
apparatus for this experiment is represented in figure 40. It con- 
sists of a large glass globe, kept filled with oxygen gas by making 
it communicate with receivers having a constant pressure. Into 
this globe issues a constant current of hydrogen gas through a very 
fine glass tube. The jet is inflamed by a feeble spark sent through 
the globe by metallic conductors, and the combustion having once 
begun, supports itself. This experiment requires much caution 
to avoid explosions ; but when we wish to observe only the fact 
of the combination of the two gases, we can safely employ the 
apparatus represented in figure 41. This is a glass tube closed 
at top with a metal stopper, which is strongly luted and which 
has a small knob projecting without the tube. A flexible metal- 
lic rod rises in the same tube by a spring, and approaches within 
a small distance of the knob. Then the tube being immersed 
in a trough of water, is filled with gas like a common receiver ; 
and being drawn partly out and wiped, a spark is given to the 
metallic cap ; it passes through the gaseous mixture, and causes 
inflammation with a loud noise. The same effect is produced by 
simple mechanical pressure ; and also by an elevation of tem- 
perature. 



Mechanical and Chemical Effects of Electricity, 105 

In the same way that we form water by the electric spark, 
we are able also to decompose it. To this end, recourse was 
had formerly to violent discharges through the liquid, which 
produced in it explosions accompanied with sparks. But the 
able and ingenious Dr Wollaston contrived to produce the same 
effect in a much more certain, easy, and beautiful manner, by 
conducting the electric current through the water by means of 
very fine platina wires, terminating in sharp points, and insulat- 
ed in glass tubes, or enveloped in resin, that they might not lose 
their electricity, except at the points themselves. It is evident 
that a very feeble electricity will, under these circumstances, 
acquire an extreme intensity, which is confined to the extremity 
of the point, and acts entirely against the single particle of 
water with which the point is in contact. Thus the electric 
current of a feeble machine, being transmitted in this way, is 
sufficient to disengage a continued stream of little bubbles, which 
being collected and tried by the electric spark, are found to be 
the two gases of which water is composed. The effect is 
rendered more certain and rapid by bringing together at the 
same time, through two opposite wires, two currents of electricity 
of different kinds. 

If the transmission is made by two very fine points, one of cop- 
per, and the other of silver, immersed in a solution of sulphate of 
copper, the first communicating with the vitreous conductor, the 
sulphate is decomposed. The copper, being separated from the 
acid, is deposited in a metallic state upon the silver wn're, and 
the other wire is dissolved. If we invert the communications, so 
as to cause the silver wire, thus covered, to communicate with 
the vitreous conductor, the deposit of copper, formed upon its 
surface, is redissolved, and the precipitation takes place upon the 
other wire. 

These beautiful experiments, and many others of the same 
kind, due also to Dr Wollaston, prove that the resinous elec- 
tricity tends to disengage oxygen from the combinations into 
which it enters, and that the vitreous electricity, on the contrary, 
favors these combinations. Of the truth of this important result 
we shall hereafter have abundant proof. 
E.&M. 14 



106 Electricity. 

Of Atmospherical Electricity and Lightning Rods, 

95. Since the discovery of the Leyden jar and electrical 
batteries, the effects of the electricity accumulated in this way, 
are found to be so similar to those of lightning, that the identity 
soon began to be suspected. Yet Franklin was the first, who, 
having observed the power of points to discharge electrified 
bodies at a distance, thought of employing this method of ren- 
dering atmospherical electricity sensible, and of securing us 
from its effects. But not having in America the means of 
making these experiments, he engaged the philosophers of Eu- 
rope to attempt them. The first who answered to this suggest- 
ion was Dalibard, a French philosopher, who built a hut at 
Marly-la-ville, upon which was erected a bar of iron forty feet 
in length, insulated at its lower extremity. A stormy cloud 
passing near the zenith of this bar, it gave sparks when the fin- 
ger was presented to it, and exhibited all the effects of conduct- 
ors electrified by our common machines. This memorable 
experiment was performed for the first time on the 10th of May, 
1752. 

Contrivances of this sort were soon multiplied ; but they all 
had a common defect, namely, the imperfect insulation of the 
base, which was liable to become wet and thus suffered the 
electricity to be dissipated. Canton remedied this imperfection 
by placing at the lower extremity of the metallic bar, a metal 
cap which covered the nonconducting support and protected it 
from the rain. By means of this improved apparatus, he found 
that certain clouds are charged with vitreous electricity, others 
with resinous; so that the electricity of the apparatus often 
changed five or six times in half an hour. Rain and snow in 
falling electrified it also, and this took place in winter as well as 
in summer. That he might not be obliged to visit it continually 
and often without success, Canton fitted to it a small and ex- 
tremely ingenious apparatus. It is composed of three little bells 
T, T^T 2 , suspended from the same metallic horizontal rod AB ; 
the middle one T by a thread of silk, and the two others by a 
metallic chain. Moreover, the bell T communicates with the 
ground by another chain attached to its under surface. Be- 



Atmospherical Electricity and Lightning Rods. 107 

tvveen these bells two metallic balls 6, b', are suspended by silk 
threads. Now it is evident that if the rod AB is made to com- 
municate with the vertical conductor which receives the elec- 
tricity of the atmosphere, this electricity will first be transmitted 
to the two extreme bells 7\, T 2 , by means of the metallic chains 
to which they are suspended. Then the little balls 6, J', will be 
attracted toward the bells and will touch them ; but they will 
be immediately repelled, and on the other hand, they will be at- 
tracted by the bell 7 1 which communicates with the ground; they 
will touch this bell, be discharged, and return to receive a new 
charge from the extreme bells. These continued oscillations of 
the little balls will produce a ringing of the bells, and we shall 
thus be apprised of the presence of electricity. This apparatus 
is called the electrical chime. 

96. But Franklin had been pursuing in America, the train of 
tfiought which first suggested itself to him, and in which he felt a 
strong interest. In the want of high buildings, it occurcd to him 
that the electricity might be made to descend from the clouds to 
the earth along the cord of a boy's kite ; and since the beautiful 
experiments of Newton upon the colors exhibited by soap bub- 
bles, this was the second time that the sports of children became 
the instruments of the most important discoveries. But Franklin 
did not foresee the extreme danger to which he was exposing 
himself. His kite was raised, and he held the cord in his hand ; 
but it gave no sign of electricity although it was near a cloud 
which appeared to be charged with lightning. Franklin began 
to fear that he was wrong in his conjectures, when, a small show- 
er having moistened the cord and increased its conducting pow- 
er, he drew sparks from it ; and he himself describes the joy 
with which he perceived the phenomenon he had thus anticipated. 
Nevertheless, if the cord had been thoroughly wet, or if it had 
been a better conductor, it is highly probable that this celebrated 
man would have paid for his temerity with his life ; and we 
should have been deprived of all he afterward achieved for 
science, philosophy, and liberty. In France, M. de Romas 
performed the same experiment in a much more perfect manner, 
having either conceived it himself, or having been led to it by 
the attempt of Franklin. He twisted a very fine iron wire with 
the cord of the kite, and that the observer might not be exposed 



62. 



108 Electricity. 

to sodden discbarges, the lower extremity of the cord was termi- 
nated by a silk string eight or ten feet in length, by which the 
kite and wire were insulated. Moreover, instead of taking 
sparks with the finger, when the observer himself receives the 
discharge, Romas obtained them by means of a metallic con- 
ductor communicating with the ground, and held in the hand 
by a non-conducting tube ; this was in fact the exciter al- 
ready described. Having thus given to his apparatus all the 
perfection which skill and prudence suggested, Romas did not 
hesitate to send it into the most highly charged clouds ; and in 
one of his experiments, during a storm which was not remarka- 
ble either for the quantity of lightning or of rain, he saw shoot 
from it for some hours jets of fire more than ten feet in length. 
" Imagine to yourself," says he to Nollet, " sheets of fire nine or 
ten feet in length and an inch in thickness, accompanied with an 
explosion louder than the report of a pistol. In less than an hour 
I obtained certainly thirty of this size, besides a great number 
of smaller dimensions. But what gave me the most pleasure was, 
that the large sheets were spontaneous, and that in spite of the 
great quantity of fire that composed them, they fell constantly upon 
the nearest conducting body. This constancy gave me so much 
confidence that I did not fear to discharge the fire with my ex- 
citer, even when the storm was the most violent ; and although 
the glass branches of the instrument were only two feet in length, 
I conducted at pleasure, without feeling the smallest shock in my 
hand, sheets of fire, six or seven feet in length." This descrip- 
tion is alone sufficient to show that such experiments are not to 
be tried without extreme care. There is one precaution which I 
cannot omit giving, because it is of the greatest importance, and 
because it applies equally to insulated metallic rods, elevated 
after the manner of Canton ; this is, to place near the lower ex- 
tremity of the bar or of the cord of the electric kite, a large iron 
bar inserted to a considerable depth in the earth or communicat- 
ing with a body of water. When the current of electricity be- 
comes strong enough to be dangerous, the explosions will take 
place upon the projecting extremity of the bar rather than upon 
any ether object more distant or even equally removed ; and by 
taking this precaution, we may enjoy the spectacle without dan- 
ger. 



Atmospherical Electricity and Lightning Rods. 109 

It being once established that the lightning is an electric ex- 
plosion, we cannot doubt that the electricity of a thunder cloud, 
like that of our machines, may be considerably weakened by 
the action of points. This inference did not escape the notice 
of Franklin ; for among the distinguished features of his genius, 
was a readiness to seize upon any useful application of new facts, 
no less remarkable than his aptitude to discover them. When 
he had no longer any doubt respecting the nature of lightning, 
it immediately occured to him to neutralize it by tbe power 
which he had discovered in metallic points, and thus he was 
led to the invention of the lightning rod. 

97. This name is given to those metallic rods, which are 
raised upon the tops of buildings, the masts of ships, &c. 
One of the extremities, which is pointed, projects into the 
atmosphere, while the other communicates with the ground. 
The effect of this apparatus is to receive or neutralize the 
electricity of the clouds, and to conduct it without an ex- 
plosion into the earth. For about fifty years, during which 
they have been in use, their utility has been proved in a great 
number of instances ; indeed their effect is evident from 
theory. When an electric cloud passes so near as to make its 
influence sensible, it decomposes the natural electricities of the 
rod, repels that of the same kind into the ground, and attracts 
that of the opposite kind to the upper extremity, where it ac- 
quires an intensity depending upon the action of the cloud. 
Hence it results that the particles of moist air situated between 
the cloud and the lightning rod, must be attracted toward the 
point with great rapidity, lose there the electricity which they 
had received from the cloud, and be violently repelled charged 
with the contrary electricity. Then flying toward the cloud, 
they neutralize the electricity of such of its particles as they 
meet with in their passage, until by this alternate motion, the 
cloud is completely discharged. There is hence reason for be- 
lieving that this discharge will take place without explosion, and 
that all conducting bodies below the lightning rod and at a small 
distance from it will be thus preserved. If, however, in an ex- 
traordinary case, this rapid discharge of the electricity should 
not be sufficient, and an explosion take place, it will infallibly 
strike upon the point, because there the reciprocal attraction of the 



110 Electricity. 

two opposite electricities is incomparably the most powerful, and 
in this the theory is fully confirmed by the fact. Soon after the 
invention came into use, the point of a lightning rod was present- 
ed to the Academy of Sciences at Paris, which had received so 
powerful a discharge that it had been melted, as fine wire is melt- 
ed by our batteries. Yet this terrible explosion, which would 
naturally have been attended with the most destructive effects 
to the house upon which it fell, did not cause the slightest injury, 
and was perceived only by the loud thunder which accompan- 
ied it. 

We are able by a very simple experiment to show 7 the effect 
of lightning rods upon a charged cloud. We suspend from the con- 
ductor of an electrical machine a linen thread, to the lower end 
of which is attached a lock of carded cotton which very well 
represents a cloud. The whole is electrified, and we present to 
the cotton, not a point, but a spherical body communicating 
with the ground ; the cotton is immediately attracted, and a 
spark is produced between the two bodies. But if, instead of a 
sphere, w r e present to the cotton a point communicating w r ith the 
ground, held at a great distance, it discharges itself insensibly 
after which it returns toward the conductor to be recharged, and 
rcdescends toward the point to discharge itself anew. We can 
suspend in this way several locks of cotton by threads of diffe- 
rent lengths, and they will be seen to fold successively upon 
each other. It is thus, probably, that the lower portions of a 
cloud, which have been discharged by a lightning rod, fold upon 
the upper parts which are still electrified. 

98. The effect and the utility of lightning rods being no 
longer doubtful, it is important to know the best method of con- 
structing them. Two conditions seem to be indispensably ne- 
cessary ; the first is, that the communication should be perfect 
with the ground and between the different metallic bars of which 
the apparatus is composed ; the other is, that the conducting 
rods should be of such a magnitude that in the most violent ex- 
plosions, the electricity transmitted shall not acquire a repulsive 
force sufficient to make it fly off. It appears from all the instan- 
ces hitherto observed, that rods of an inch square, or an assem- 
blage of large iron wires of equivalent dimensions, are perfectly 
sufficient for this purpose. 



Atmospherical Electricity and Lightning Rods. Ill 

If these conditions are strictly observed, theory as well as 
experiment tends to show that there is no danger from being in 
the vicinity of a lightning rod or even from being in contact with 
it, the electric charge always choosing the best conductors, and 
consequently following the metallic rods rather than any neigh- 
bouring body of less conducting power. Thus, when an iron w r ire 
is made to pass through a package of gunpowder, we may safely 
transmit, by means of this wire, any electric discharge which 
is not sufficient to melt it, or to heat it to such a degree as to in- 
flame the powder. Also, let a bird stand on one of the conductors 
of the machine, during the discharge of a battery, it will not be 
affected, although the course of the electricity comes in contact 
with it. Finally, by surrounding the body with a metallic wire, 
the extremities of which are held in the hand, we may safely 
discharge the largest batteries through this wire, if, like the bird, 
we are insulated on the line of communication. 

In these experiments we sometimes feel a slight, instantane- 
ous shock, but incomparably weaker than that produced by the 
discharge of the battery. The cause of this shock is, that the 
electricity accumulated in the battery is not transmitted with 
perfect freedom, and is not discharged in a single indivisible in- 
stant, however good the conductor which is presented to it. In 
this case, it acts by influence upon the natural electricities of the 
bodies in contact with this conductor, and produces in them a 
separation which continues for an instant. The equilibrium is 
immediately restored, but the sudden alternation of these two 
states produces a slight disturbance in the bodily organs. From 
this it will be seen that the effect will be the more feeble accord- 
ing as the communication between the two surfaces of the battery 
is effected with larger and more perfect conductors. 

To show the truth of these remarks, we insulate a cylindrical ^. An 
conductor AB, and place it in contact with the exterior surface of 
a battery which communicates with the ground. Near one of the 
extremities of this conductor, we place another conductor A'B', 
also insulated, but separated from the first by a small space. At 
the moment of the discharge, a spark will be seen to escape 
from the first conductor to the second, and an electroscope, pla- 
ced upon the latter, will suddenly rise and fall. If we terminate 
this second conductor by the apparatus represented in figure 4 1 , 




1 1 2 Electricity. 

making its cap communicate with A'B', and its rod with the 
earth, the lateral discharge will inflame the gaseous mixture 
contained in it. 

The only danger to be apprehended from lightning rods, 
arises, therefore, from this lateral discharge, which may be 
diminished at pleasure by increasing the dimensions and the 
conducting power of the rod. Both theory and experience 
teach us that this shock is incomparably less than that of the 
direct discharge; and if it even becomes sensible, what would 
the discharge itself have been, if there had been no metallic 
conductor to convey it to the ground ? t 

99. It has sometimes happened in a thunder storm, that men 
and brute animals have fallen dead at the instant of an explosion, 
although they were far distant from the place of the discharge. 
This phenomenon admits of an easy explanation. Imagine a 
cloud highly electrified, and of which the two extremities incline 
toward the earth ; they will repel from the ground the electric- 
ity of the same kind with that belonging to the cloud, and will 
attract that of the contrary kind. If from any cause a discharge 
should suddenly take place at one of these extremities, the equi- 
librium will be immediately reestablished at the point of the 
earth under the other extremity ; and this restoration of the 
equilibrium, if the discharge is very powerful, may be sufficient 
to occasion the death of animals exposed to it. This phenome- 
non is called the electrical returning stroke. Its effect may be 
illustrated by the following experiment. 

Suspend a living frog by a silk cord, at some distance from the 
conductor of an electrical machine, as represented in figure 44 ; 
let there be attached to one of its legs a very light, flexible wire, 
communicating with the ground ; then put the machine in motion, 
and as the electricity is developed, from time to time, draw 
sparks from the prime conductor, by presenting to it a metal rod 
terminated by a hemisphere. At each explosion, the frog will 
be seen to quiver, although he is not in the arc of communica- 
tion ; the natural electricities, decomposed by the influence of 
the electrified conductor, suddenly unite each time that this in- 
fluence is destroyed, and excite a commotion in the organs of 
the animal. 

t See subjoined note on the best form of lightning rods. 



Atmospherical Electricity. 1 1 3 

These effects take place even after death ; to observe them in 
all their activity, the frog should be killed suddenly ; it is then to 
be skinned and prepared, as represented in figure 45. The irritabil- 
ity is such that the muscular contractions are produced by the 
influence of a powerful machine, even at the distance of thirty or 
forty feet. This phenomenon, so simple in itself, shows that the 
muscular organs of frogs are electroscopes of an extreme sensi- 
bility. It will be seen in one of the following sections, that this 
sensibility has been the occasion of one of the finest discoveries 
ever made in natural philosophy. 

100. We have thus far studied atmospherical electricity only 
in the violent and transient state in which it appears in thunder 
storms ; but by increasing the delicacy of the instrument em- 
ployed to make it sensible, we may hope to discover it when it 
would be inappreciable by ruder instruments. For this purpose 
we arm the straw or gold leaf electroscope with a pointed me- 
tallic rod, whose lower extremity is screwed to the end of the 
stem which communicates with the straws. This rod is com- 
monly about forty inches in length, and is composed of several 
pieces sliding upon each other, that their length may be varied 
at pleasure. By the aid of this instrument we discover that the 
atmosphere when pure is in a constant state of vitreous elec- 
tricity ; but clouds or vapour in the smallest quantity affect 
this state. For a stronger reason, it changes when the atmos- 
phere is more violently disturbed, as in the case of strong 
winds, rain, snow, hail, and tempests. 

The electroscope of Coulomb, so convenient and delicate in 
all other experiments, is equally well adapted to the purpose of 
observing these phenomena. To this end we have only to put 
its fixed stem in communication with an insulated metallic rod, 
like that attached to the straw electroscope, and the smallest va- 
riations that take place in the atmosphere, will become sensible by 
their influence upon the moveable disc, especially if we begin by 
charging it with a small quantity of a known electricity. Coulomb 
even dispensed with the rod or permanent conductor, and fixed a 
small metallic sphere at the end of a stick of sealing wax, which 
served to insulate it, and he attached this stick to a wooden pole 
five or six feet in length. Then, when he wished to try the elec- 
tric state of the atmosphere, he held the pole up in the air, 
E. &M. 15 



114 Electricity. 

touching for a moment the small sphere with a metal rod or a 
simple wire held in the hand. Afterward, withdrawing the rod 
or wire, he presented the sphere, which, on account of its being 
insulated, preserved the electricity it had acquired, to the move- 
able circle of the electroscope, upon which it immediately acted. 
This experiment always succeeded when the electroscope was 
in an open place, where the air had free access to it, and where 
the electric state of its strata situated near the ground is not 
affected by the vicinity of conducting bodies, as trees and the 
walls of buildings. 

101. The intensity of this constant electricity increases as 
we ascend into the atmosphere ; and thus, in order to render it 
more sensible, Saussure proposed to throw into the air a heavy 
ball attached to a very fine wire, the lower end of which, being 
twisted about the stem of the electroscope, adheres to it slightly 
by its own spring. When the wire is extended by the motion of 
the ball, it gives to the electroscope the same kind of electricity 
with the stratum of air to which the ball has risen. But by con- 
tinuing to move after the wire is entirely taken up, it detaches 
itself from the stem of the electroscope, which thus remains in- 
sulated and charged with the electricity it had acquired. 

102. When M. Gay-Lussac and myself ascended in a bal- 
lon for the purpose of making experiments, to be described here- 
after, when we come to treat of the magnetism of the earth ; we 
also collected the electricity of the atmosphere by methods similar 
to that of Saussure. A wire 150 feet in length was suspended from 
our car, being stretched by the weight of a metallic ball. W r e were 
by this means in communication with a stratum of air situated 
150 feet below us. The atmospheric electricity collected at the 
top of this wire, very sensibly affected the electroscope ; and 
being tried with sealing wax, it was found to be resinous, al- 
though the weather was perfectly fair. 

This result appears to contradict that of Saussure, which has 
been since confirmed by different observers ; but the contradic- 
tion is only apparent; the two results are found in fact to agree. 
To prove this agreement, let us represent the wire in question 
Fig. 47. by ^j^ . anc [ a t its two extremities suppose two horizontal planes 
separating the atmosphere into three strata, one above the wire, 
one comprehended between its extremities and the other below 



Atmospherical Electricity. 115 

the wire. Now suppose that the atmosphere is really in a state 
of vitreous electricity increasing with the height. It must be 
admitted that this electricity is feeble and that its increase is in- 
considerable, especially for the distance of 1 50 feet. This being 
premised, let us first consider the action of the two extreme stra- 
ta. We do not now refer to their action by contact ; for this 
must employ a certain time in order to be transmitted, but of the 
influence at a distance of their free electricities upon the natural 
electricities of the wire. The upper stratum S, which is in the 
vitreous state, attracts the resinous electricity of the wire with 
a force which may be expressed by -f- R, and repels the vitre- 
ous with a force which may be denoted by -f- V. The lower 
stratum S' will do the same in the opposite direction ; but its 
action will be more feeble, for the intensity of the vitreous 
electricity is supposed to increase with the height. Let, then, 
r and v be the two forces which it exerts. From this it is evi- 
dent that the resinous electricity of the wire will be attracted 
toward the upper part of the wire with an excess of force equal 
to R — r, and the vitreous electricity will be repelled toward 
the lower extremity with an excess of force equal to V — v. 
Therefore, to us who observed the electricity at the upper part 
of the wire, it ought to be resinous. To Saussure, who examin- 
ed it at the lower extremity, it ought to be vitreous. 

We have not considered the action of the intermediate stra- 
tum AB, upon the electricities of the wire. If this stratum were 
uniformly electric throughout its whole thickness, its action above 
and below each half of the wire would counterbalance each 
other, and there would result no decomposition of the natural 
electricities of the wire. But the vitreous state increasing with 
the height, it is evident that the united actions of all the particles 
of the stratum will produce a resultant of the same nature with 
the action of the upper stratum, so that this action is thus augment- 
ed ; and the total effect will also be augmented, if the thick- 
ness of this stratum is so great that its action may be compared 
with those of the upper and lower strata of the atmosphere. 

103. We give another experiment, due to M. Hermann, which 
is explained on the same principles. A very sensible gold leaf 
electroscope is firmly fixed at a certain height in the air, the 
weather being fair. It gives no sensible signs of electricity. 



116 Electricity. 

We carry into the stratum of air, a few feet only above the 
electroscope, a wire or any other conducting substance, placed 
horizontally at the extremity of a nonconducting rod ; and after 
having held it for some time in this stratum, we suddenly bring 
it down till it touches the electroscope ; the leaves of which im- 
mediately diverge with vitreous electricity. On the contra- 
ry, if we carry the insulated conductor into a stratum below the 
electroscope, and, after suffering it to remain for a time, raise it 
with a quick motion, it gives to the electroscope the resinous 
electricity. 

These phenomena are explained on the supposition that the 
moveable conductor takes each time the degree of electricity 
which belongs to the stratum in which it is placed. When brought 
back so suddenly as to prevent its state from being entirely 
destroyed by the contact of the particles of air through which it 
passes, it communicates this state to the electroscope ; if it comes 
from above, it brings with it an excess of vitreous electricity ; if 
from below, it is attended with a deficiency of this same elec- 
tricity. Let -f- E be the quantity of free vitreous electricity 
which the conductor must have in order to preserve an equi- 
librium in the stratum of air where the electroscope is placed ; 
so that when at + E the particles of air of this stratum neither 
add any thing to it, nor take any thing from it. It is carried 
into a higher stratum where it takes E + dE; dE denoting 
the small excess of electricity which it acquires there. If it be 
then rapidly brought back to the stratum of the electroscope, it 
will be too much electrified by the quantity d £, and will com- 
municate this to any body with which it may come in contact; 
it will therefore communicate it to the electroscope, if placed in 
immediate contact ; and the leaves will diverge with vitreous 
electricity until by the contact of the air, this excess is destroy- 
ed. On the contrary, when the insulated conductor returns from 
a lower region, it possesses the electricity -\- E — d E, less than 
E by the quantity d E. If it be brought in contact with the 
electroscope, the instrument will share this state ; and the quan- 
tity of vitreous electricity which it then possesses will be insuffi- 
cient to place in equilibrium the influence of the surrounding 
atmosphere, and its natural electricities will be decomposed. 
But the portion of vitreous electricity which this decomposition 



Atmospherical Electricity. 117 

renders free, will not cause the leaves to diverge, because its 
repulsive force will be wholly employed to compensate that of 
the exterior electricity E. The repulsive force, therefore, of 
the resinous electricity only will exert itself, because there is 
nothing to compensate it ; and the gold leaves will diverge in 
virtue of this electricity until it has been removed and neutral- 
ized by the immediate and successive contact of the particles of 
air. Experiments of this sort present the singular circumstance 
of an indefinite medium, the air, the particles of which are each 
charged with an excess of electricity of the same kind, so that 
the entire mass of the medium is penetrated with it in a propor- 
tion that varies with the height. Hence the different parts of 
this medium cannot be at rest except by a combination of their 
repulsive forces with their gravity; and the same condition applies 
also to the conductors surrounded by them. Thus, for all these 
conductors, the electric equilibrium will not exist when their natu- 
ral electricities are completely neutralized, but only when they 
possess an excess of whichever electricity belongs to the stratum 
in which they are situated ; and this excess is vitreous in a pure 
atmosphere. If they possess a greater excess of this same elec- 
tricity, they will act solely in virtue of this excess upon each 
other, and also upon the particles of the surrounding air ; they 
will therefore mutually repel each other. If, on the contrary, 
the excess of electricity which they possess is less than that 
which they would naturally take in the stratum where they are 
placed, the whole mass of the medium will act upon each one 
of them in virtue of this difference ; and their natural electricities 
will be decomposed sufficiently to complete what they want of 
the electricity of the medium. On account of this addition, they 
will repel the medium as much as the medium repels them, and 
will suffer from it no action. But they will act upon each other 
with the excess they have acquired of the opposite electricity, 
and if the medium is an indefinite fluid composed of particles 
capable of being electrified by contact, the excess will gradually 
be dissipated in space. Many curious experiments might be 
made to determine the laws of electrical equilibrium in circum- 
stances so different from those we are in the habit of consider- 
ing. 



1 1 8 Electricity. 



Of Electrical Light. 

104. The light which is observed during an electric explo- 
sion, was for a long time considered by philosophers as a modi- 
fication of the electric principle itself, which they supposed to 
possess the quality of becoming luminous at a certain degree 
of accumulation. But by observing the light which is disengag- 
ed from the air by mechanical pressure, we are led to think 
that the electric light may have a similar origin, and be simply 
the effect of the pressure of the air by the electric explosion. 
This is rendered extremely probable by a critical examination 
of the experiments that have been performed relating to this 
subject.! According as the air, which is traversed by the 
charge, is more or less dense, or as the shock itself is more or 
less powerful, the colours produced vary from the softest violet 
to the most dazzling white. This effect takes place in a vacu- 
um of the air pump, and even in that of the barometer. But 
what is such a vacuum but a space containing the vapour of 
water or that of mercury, which, as well as air, may disengage 
heat when sufficiently compressed. 

105. Free electricity is attended also with two other effects 
which h'ave been regarded as belonging to its phyical constitution. 
The first is the sensation, similar to the touch of a spider's web, 
which electrified bodies produce, when brought near to any 
part of the naked skin. The second is the odour of phosphorus 
which is very sensibly emitted by the electric points when they 
are presented to the organs of smell. But the commotions pro- 
duced by the Leyden jar and electric batteries, prove that the 
electricity when in action, violently shocks the organs and excites 
in them strong muscular contractions. We shall see hereafter 
other examples of this property. Now, when an electrified con- 
ductor is presented near any part of the body, there takes 
place in this part a decomposition of its natural electricities, and 
that which is of a contrary nature to the electricity of the con- 
ductor, is condensed at the part nearest to the conductor. May 

t See Biofs Traite de Physique. Tom. ii. p. 459. 



Electrical Light, 111) 

not this internal motion, this departure of one kind of electricity 
or the introduction of the other, produce in us a certain sensa- 
tion ? And must not the contact of the air alone, which is renew- 
ed and electrified upon the parts of the skin where the electricity 
has become free^ excite there some commotion ? If this be the 
fact, there is no reason for going out of our way to imagine par- 
ticular causes to produce the effect in question ; and there is, con- 
sequently, no propriety in considering these physical properties 
as belonging to the nature of the electricity. 

106. By varying the direction and the scintillations of the 
electric light, many interesting results have been obtained. I 
shall confine myself to describing two which seem to indicate 
a physical difference between the two electricities. 

We arm the prime conductor of an electrical machine, or 
one of the secondary conductors attached to it, with a metallic 
point projecting into the air. We then arrange the rubbers 
in such a way, as to charge these conductors successive- 
ly with the vitreous and resinous electricity. If the experi- 
ment is made in the dark, we observe, in the first case, at the 
extremity of the point, a conical brush of light attended with a 
very sensible rustling noise ; in the second only a luminous point 
is seen unaccompanied with any noise. 

107. We suspend by a silk thread a piece of pasteboard, as Fig. 48. 
a playing card, the two surfaces of which are placed in contact 

with two metallic points, directed parallel to each other, but not 
directly opposite at the point of contact. One of these points is 
made to communicate with the exterior surface of a Leyden jar 
which is held in the hand, and we touch the other point with the 
knob of the jar ; the discharge is from one point to the other, pass- 
ing through the card. Now we observe that the place where the 
card is perforated, is always situated directly opposite the point 
which communicates with the resinous surface of the jar ; and if 
the experiment is made in the dark, at the moment of discharge, 
a spark will be seen darting over the surface of the card in con- 
tact with the vitreous conductor ; while the surface which touch- 
es the resinous conductor remains dark. We may preserve the 
traces of this passage by painting the two surfaces with Vermil- 
lion, which is found to be altered only on one of them. 



1 20 Electricity. 

This phenomenon and the preceding are very well explained 
by supposing that the air affords a much easier passage to the vit- 
reous electricity than to the resinous. Then a point charged 
with vitreous electricity will dissipate it suddenly, while if 
it is charged with the resinous electricity, the discharge must 
take place by the successive contact of the particles of air, 
which, touching the extremity of the point, carry off the elec- 
tricity from it. No light will be produced, therefore, except at 
this extremity ; accordingly, in the case of the card, the elec- 
tricity of the vitreous point only darts into the air to combine 
with the electricity of the other point ; by taking that course 
which offers the least resistance, gliding at first along the surface 
of the card and piercing it at the moment it is opposite to the 
other point, the attraction being then the most powerful. M. 
Tremery, who first explained the phenomenon in this way, con- 
trived to weaken the influence of the restraining force, by di- 
minishing the density of the interposed air ; and this he did by 
repeating the same experiments under the receiver of an air 
pump. He thus found that the hole in the pierced card ap- 
proaches nearer to the middle of the interval between the two 
points, according as the surrounding air becomes more rare, and 
thus opposes a less resistance. This result seems to agree with the 
supposition of an unequal restraining power being exerted upon 
the two electricities. We shall hereafter make known phenomena 
which prove the existence of a similar inequality in other sub- 
stances besides the air. But this inequality is not sensible, 
except for electric charges having a very feeble repulsive force ; 
and it is very difficult to conceive how it can exist in the air, 
even for the strongest charges, when all other phenomena seem 
to indicate that the resistance opposed by the air, to the expan- 
sion of the electricity, arises solely from its pressure. It 
would be well, therefore, to repeat the experiments under new 
circumstances, for instance, in different media, that, if possible, 
these apparently contradictory facts may be reconciled. 



Different Methods of developing Electricity, 121 



Of the different Methods of developing Electricity. 

108. To study the different properties 'of the electric fluids 
and establish their respective characters, it is sufficient to have 
some certain and easy method of exciting them. This is com- 
monly found in friction ; and as its effects admit of being indefi- 
nitely augmented, in the former part of this treatise we have 
described and used no other method. But it now becomes ne- 
cessary to make known other means of acting upon bodies, by 
which their natural electricities may be separated ; for it is 
only by experiments of this sort that we can discover in what 
manner the two electric principles are connected with the natu- 
ral constitution of bodies. 

In the first place, I have said, in speaking of friction, that 
apparently the most trifling circumstance determines a body to 
take one rather than the other electricity, when rubbed against 
another body. For example, if we rub against each other two 
silk ribbons AB, A'B\ cut without any distinction from the same 
piece, placing them crosswise in such a way, that one of them 
AB shall rub successively throughout its whole length, while 
A'B' is rubbed only in the part C, the former always takes the 
vitreous, and the latter the resinous electricity. In this case, the 
electricity is determined merely by the manner of the friction. 
But the higher or lower temperature of one of the two bodies has 
an important effect upon the kind of electricity which it acquires, 
as is proved by Bergman. In the preceding experiment, for 
instance, if the ribbon AB, which rubs successively throughout 
its whole length, is at first heated to a high degree, and the fric- 
tion is not continued so long as to reduce it to nearly the same 
temperature with A'B', this circumstance will have more effect 
than the manner of the friction, and AB will now take the resin- 
ous, and A'B' the vitreous electricity. After the ribbon AB has 
become cold, or when the two ribbons have come to the same 
temperature, things will return to the state first described ; and 
in the passage from one of these states to the other, there will 
be a point at which a state of indifference will be manifested. 
To conduct experiments of this sort with all possible delicacy, 
E.frM. 16 



122 Electricity. 

and to be able to follow with certainty all the variations of the 
electric state, it is necessary, after each operation, to present in- 
stantly each of the bodies upon which we operate, to a very 
sensible gold leaf electroscope, merely touching the knob with 
the body whose electric state we wish to know. We may 
also employ to advantage Coulomb's silk thread electroscope. 
From a series of curious experiments made with this instrument 
by that ingenious philosopher, he thought he had discovered a gen- 
eral law, which, although somewhat indefinite in its expression, 
seems nevertheless to harmonize with too many facts not to in- 
clude the germ of a general truth. It is enunciated as follows ; 
when the surfaces of two bodies are rubbed together, that body, 
the integrant particles of which are least removed from each 
other, and which vibrate least about their natural positions of 
equilibrium, seems, by this very circumstance, to be the more 
disposed to receive the vitreous electricity. This tendency is 
increased if the surface suffers a momentary compression. Re- 
ciprocally, that body, the particles of which are most removed 
asunder by the roughness of the other, or from anj^ other cause, 
is on that account the more disposed to take the resinous electric- 
ity. This tendency is increased when there is a real dilatation 
of the surface. Heat, by separating the particles of the body, 
appears to act in this way, and to dispose it to the resinous state. 
It is necessary to remark that these conditions are not presented 
by Coulomb as absolute, but merely as relative ; that is, they 
simply dispose bodies to such an electric state, but do not deter- 
mine them to it necessarily ; for doubtless the very nature of 
the bodies thus rubbed, has an influence upon the phenomenon ; 
but this influence Coulomb did not attempt to estimate. 

109. The preceding principle applies very well to an exper- 
iment made by M. Libes long after Coulomb had advanced 
these ideas. The experiment is as follows. We take a disc of 
metal which is held insulated by a glass handle, and press it 
upon some gummed taffeta, either simple, or consisting of seve- 
ral thicknesses. The gum with which the taffeta is covered, is 
capable of being compressed, and it is on this account that it 
adheres to bodies, the asperities of which leave their impressions 
upon its surface. According to Coulomb, it is now in a condi- 
tion to facilitate the developement of the vitreous electricity ; 



Different Methods of developing Electricity. 123 

and we in fact find this kind of electricity, when we remove the 
disc ; and the disc possesses a corresponding excess of resinous 
electricity. The effect is more marked according as the press- 
ure is greater. It ceases when the taffeta has lost its glutinous 
quality which rendered its surface easily compressible. Fric- 
tion is not concerned in this phenomenon ; for if, instead of 
pressing the disc upon the taffeta, we lay it lightly upon its sur- 
face, and move it backward and forward in order to produce 
friction, the disc takes the vitreous electricity, and the taffeta the 
resinous ; a precisely opposite effect to that produced by pres- 
sure. 

110. The curious remark of M. Libes has been extended by 
M. Hairy to several mineral substances, with this striking pe- 
culiarity, that some of them take the electric state with the 
slightest pressure, and afterward obstinately retain it. For in- 
stance, the rhomboidal carbonate of lime, commonly called Ice- 
land spar, becomes electric when merely pressed for an instant 
between the dry fingers ; its electricity is quite sensible, and of 
the vitreous kind, which it retains with much force ; for it does 
not yield it to a conducting body which communicates with 
the ground, nor even when it is immersed in water. Other 
minerals possess this property in a less degree ; and some ap- 
pear to be altogether destitute of it. But M. Becquerel has 
shown that the exception is only apparent, and is owing to this, 
that the bodies in question have not, like the first, the property 
of retaining the electricity which they have once acquired ; and 
hence, to render it sensible, it is necessary to insulate them during 
the contact. For this purpose, he fixes the substance to be ex- 
amined to one end of a glass rod, the other end being terminated 
by a handle of dry wood, in order that it may be held in the 
hand without being electrified by friction. This little apparatus 
is then to be left for some time without, being touched ; in order 
to ascertain that it is not electrified, he next presents it to the 
disc of Coulomb's electroscope, charged with a known electri- 
city ; and when assured that it is perfectly neutral, he presses 
the mineral with the finger, or upon any solid body, whether in- 
sulated or not. Proceeding in this way, he found that not only 
minerals, but all substances of whatever nature,, being insulated 
and pressed against each other, come from the pressure in diffe- 



124 Electricity. 

rent states of electricity, the one with an excess of vitreous, the 
other with a corresponding excess of resinous electricity. If 
one only of the two bodies is insulated, that only preserves the 
electricity which the pressure has given it, and the other parts 
with it to the ground, unless its substance happen to be of a non- 
conducting or imperfectly conducting nature, which permits the 
the electricity of its surface to fix itself by the decomposition of 
the natural electricities of the interior lamina?. This appears to 
be the case with the Iceland spar. The absolute intensity of 
the effects is different in different substances ; and in some, they 
are so feeble that they can be made sensible only by particular 
precautions ; the most essential of which is to give to the bodies 
employed, the form of small discs of a few hundredths of an inch 
in diameter. Their electric properties are also very much 
heightened by being warmed. Some substances, as tinder and 
elder pith, manifest no sensible effects without having their tem- 
perature raised. 

111. It will be seen in the following section, that according 
to a very beautiful discovery of Volta, bodies of whatever kind 
being placed in contact, are found, upon being separated, to have 
different electrical states ; but the phenomena observed by M. 
Becquerel seem, by their intensity and by many circumstances 
which attend them, to be of another kind. For instance, if we 
press an insulated disc of cork upon the palm of the hand, or 
the living hair upon a wooden table or upon an orange peel, and 
after having withdrawn it, bring to it the knob of the gold leaf 
electroscope, two or three successive pressures, and sometimes a 
single one, will be sufficient to give to the leaves a considerable 
divergence; while it is necessary to arm the electroscope with 
a condenser to render sensible the electricity developed in it by 
simple contact. Moreover, the facility with which substances 
admit of being compressed and afterwards restore themselves, 
very much favours this developement of electricity by pressure. 
Much is excited, for instance, by pressing an insulated disc 
of cork upon a mass of leaves stitched one upon another. The 
imperfect liquids which are capable of being compressed and of 
restoring themselves afterward, are equally adapted to produce 
these effects, as may be seen by pressing the insulated disc of 
cork upon some oil of turpentine thickened by boiling, which 



Different Methods of developing Electricity. 1 25 

forms a sort of varnish of imperfect fluidity. M. Becquerel has 
remarked, also, that in these experiments, as in that of M. 
Libes, the electricity developed by pressure becomes more in- 
tense according as the substances adhere more closely to 
each other, when pressed together, and require a more sensible 
effort to separate them. Generally, this developement appeared 
to him to be modified by an infinite number of circumstances, 
such as the polish of the surfaces, the more or less moist state 
of the air to which they are exposed, and their more or less 
recent separation. 

112. M. Dessaignes long ago made known a fact which 
seems to have much analogy with the preceding ; it consists in 
this, that if a rod of glass or sealing wax be immersed in mercu- 
ry, it usually comes out electrified, whether it is entirely immers- 
ed, or merely placed upon the surface of the liquid, or is em- 
ployed to give a smart blow to this surface. The most simple 
means of verifying this fact, is to present the rod, after it is with- 
drawn from the mercury, to the disc of Coulomb's electroscope, 
previously charged with a known electricity. The effect is par- 
ticularly remarkable when gum lac is used, for the electricity 
which it acquires by a single immersion is stronger than that 
produced by friction. 

M. Dessaignes has remarked variations in the nature and 
intensity of the electricity acquired by the immersed rod, which 
seemed to him to depend on the state of the atmosphere, as to 
humidity, temperature, and pressure: If the electroscope made 
use of have sufficient sensibility to indicate perfectly their vari- 
ations, we might with some probability attribute them solely 
to the hygrometric state of the surface of the rods, which, ac- 
cording to the relation it bears to that of the surrounding atmo- 
sphere, would cause them to emit or condense vapour ; in fact* 
Volta and several others after him, have affirmed that aqueous 
vapour, in forming, absorbs vitreous electricity. 

113. The sudden separation of the parts of bodies, when ob- 
served in the dark, is often accompanied with a more or less 
permanent disengagement of light. This effect is apparent, for 
instance, when a piece of sugar is crushed, even though the su- 
gar is immersed in water ; in this case it is sudden like the blow 
which produces it. The phosphorescence is more permanent in 



12G Electricity. 

chalk when pounded with a hammer. May it not be that the 
light thus disengaged, indicates, when it is sudden, a decomposi- 
tion of the natural electricities ? For example, when we separate 
rapidly in the dark, the leaves of a piece of Siberian mica the 
extremities of which have been previously fixed to nonconduct- 
ing rods, a vivid bluish flash of light is seen upon the separating 
surfaces. Now if we present these surfaces to the electroscope 
after their separation, it is found, as was observed by M. Bec- 
querel, that one is electrified vitreously, and the other resinously. 
Why may it not be the same in other cases of violent separation ? 
Quantities of electricity too small to be appreciated by our best 
electroscopes, are yet perhaps capable of disengaging by their 
developement a visible light. 

The account which I have now given of these various ex- 
periments, shows that the developement of the electrical prin- 
ciples is still but imperfectly understood ; but we must, at the 
same time, perceive that it affords one of the finest subjects of 
physical enquiry. 



Of the Developement of Electricity by simple Contact, 

114. We. now proceed to consider the developement of elec- 
tricity by simple contact. This branch of Natural Philosophy, 
which dates only about thirty years back, presents the contrast of 
a great discovery, resulting from an accident, and of one still 
greater, made directly and carried out by the most rigorous 
inductions and experiments. 

It was about the year 1789 that the first phenomena of this 
sort presented themselves. Galvani, professor of Natural Phi- 
losophy at Bologna, instituted some inquiries on the excitabili- 
ty of the muscular organs by electricity. He employed in his 
experiments frogs recently killed and skinned, of which he di- 
vided the spine in order to insulate and lay bare the lumbar 
Fig. so. nerves# That he might manage them conveniently, he introduc- 
ed into the remaining part E of the spine, a copper wire bent 
in the form of a hook. It accidently happened one day that 
several frogs were suspended by these copper hooks from the 



Developement of Electricity by simple Contact, 127 

iron balcony of a terrace ; at that instant their feet and legs, 
which also lay in part upon the iron, became spontaneously 
convulsed ; and the effect was the same at every new contact. 
Galvani perceived the importance of this phenomenon, and set 
himself to determine its essential circumstances. He saw, in 
the first place, that instead of holding the frog by the hand, it 
might be laid on an iron plate, and that applying to this plate 
the copper hook, the convulsions still took place. He next per- 
ceived that the whole was reduced to the establishing a commu- 
nication between the muscles and nerves of the frog by a metal- 
lic arc. He observed that the convulsions still took place, when 
this arc was of a single metal, but that they were then very rare 
and very feeble, and that to render them strong and permanent, 
it was necessary to employ two different metals in contact. This 
condition being fulfilled, the communication might be completed 
by any substances whatever provided they were conductors of 
electricity. He introduced into the chain of communication 
other animal substances, and even living persons who held each 
other by the hand ; convulsions still took place. Now Galvani 
had recently observed, that the electricity developed by the 
ordinary methods produced similar effects upon the organs of 
frogs, when they were exposed to its influence. A most ev- 
ident analogy seemed therefore to lead directly to the conjecture 
that the convulsions produced by the contact of the heterogene- 
ous metals were also the effect of some electrical current which 
this contact developed. Nevertheless he did not draw from it 
this simple conclusion ; he thought he saw in it the extraordinary 
effect of a new source of electricity, which he called animal 
electricity, and which, existing originally in the muscles and 
nerves, circulated when these parts were placed in communica- 
tion by a metallic arc, or hy any good electrical conductor. 
Galvani vainly attempted to compare this action to that of the 
Leyden jar; but on looking at the work itself in which this hy- 
pothesis is advanced,! it is apparent that he was not acquaint- 
ed with the true theory of electrical influence, and that, explain- 
ing the circumstance in this way, he was led to adopt theories 



t De Viribus Electrieitatis in Motu Musculari Commentarivs. 



1 28 Electricity. 

that had little of reason or ingenuity to recommend them. We 
are thus compelled the more to admire the rare sagacity and true 
genius by which he seized, as if by divination, and varied with 
so much skill, the extraordinary phenomenon of the seemingly 
spontaneous convulsions which he had accidently observed. 

When these new facts were made known in Italy, they excit- 
ed general admiration, and all were inclined to favor the views 
ofGalvani. But the celebrated Volta of Pavia had no sooner 
repeated the experiments, than he drew from them altogether 
different conclusions ; and it may be said that accident itself, by 
making known these phenomena subsequently to the sensible 
effects of artificial electrical influence, had thus sought to indicate 
their true source. Therefore Volta had no doubts with respect 
to its nature. Conceiving that the cause of these motions, what- 
ever it was, must be very subtile, since they were produced in- 
dependently even of the will of the observer, he set himself to 
determine by exact experiments the precise quantity of electri- 
city necessary to excite convulsions in the organs of frogs, by 
causing a discharge to pass through them. He thus discovered 
that this quantity was exceedingly minute, and scarcely sufficient 
to produce a sensible divergence in the straws of the delicate 
electroscope which he made use of. This result being obtain- 
ed, he compared it with the other fact established by the ex- 
periments of Galvani himself, that the contact of two or more 
heterogeneous metals was, or at least thus far appeared to be, 
necessary to excite the convulsions ; and he hence drew this 
conclusion, that the mere contact of the heterogeneous metals was 
the unperceived circumstance, which caused the sudden devel- 
opement of electricity. In following out this truly fundamental 
idea, Volta collected under one point of view all the experiments 
hitherto made by Galvani, and he pointed out the means of re- 
producing the same effects in a certain manner, and with the 
highest degree of energy. In making use of different metals, he 
observed that the best was zinc placed in contact with silver or 
copper, although the convulsions might also be produced by an 
arc composed of any two metals whatever. 

1 1 5. From the preceding observations, w T e infer that the best 
preparation for repeating the experiments of Galvani is the fol- 
lowing. Take a frog and separate the hind legs and a part of 



Developement of Electricity by simple Contact. 1 29 

the spine ; next remove the flesh and all the parts which cover 
the lumbar nerves, denoted by JVJV. Then enclose these nerves FJ 
in a small strip of copper or zinc ; place the frog, thus prepared, 
upon a nonconducting support, for instance, upon a pane of glass 
varnished with gum lac ; and, taking a piece of any other metal, 
bent into the form of an arc of a circle, place one of its extremities 
upon the armature of the nerves, and the other upon the muscle 
of the thighs ; the convulsions will immediately take place, not 
only in the leg which has been touched, but also in the other. 
The frog retains its susceptibility of these motions some time af- 
ter death; and it retains it the longer according as it has been less 
excited. When beginning to decline, it may be restored by the 
application of such stimulants as tend to increase animal irrita- 
bility. The same is to be observed also with respect to the 
convulsions which are produced in the organs of frogs by the 
influence at a distance of common electricity; and the only con- 
clusion to be drawn from all we have said, is, that these organs, 
when fresh, sensibly indicate the smallest discharges of elec- 
tricity. 

116. Guided by the fundamental idea which thus revealed 
the secret of this kind of action, Volta ascribed to the same 
cause several phenomena of sensation, which had not as yet 
been attended to, doubtless because they stood alone, but 
which, when accurately examined, are found to refer themselves, 
in the most evident manner, to the action of several metals in 
mutual contact. For example, he recalled to mind an experi- 
ment described in an old work, entitled, Theory of Pleasure, 
and which is extremely well adapted to show this influence. 
Take two pieces of different metals, one of silver or copper, and 
the other of zinc, for instance. Place one of these pieces above, 
and the other below the tongue, in such a manner that they maj^ 
project a little beyond the tip of the organ. As long as the 
pieces are separated from each other by the tongue, no effect is 
produced. But when they are made to touch each other, a pe- 
culiar taste is perceived very much resembling that of the sul- 
phate of iron. Here, according to Volta, electricity is de- 
veloped by the mutual contact of the two pieces ; and the sur- 
face of the tongue, which is covered with nervous papillas of an 
extraordinary sensibility, serves as a conductor. Sometimes, 
E.fyM. 17 



130 Electricity. 

also, the excitation is transmitted to other nerves; and if the 
person is in the dark, he perceives a flash of light in his eyes. 
All the sensible parts of animals are capable of being affected by 
such an arrangement. This susceptibility has become in anat- 
omy the certain and delicate means of discovering the most sub- 
tile nervous fibres in different parts of the organs of animals. 

117. Galvani endeavoured to support his hypothesis of an 
animal electricity in opposition to the Pavian professor; he 
urged as an objection to the theory of the latter, the convulsions 
excited by an arc of a single metal, and endeavoured to vary 
the circumstances of this experiment. For instance, after a 
frog is quickly prepared in the manner we have just described, 
if it be immediately laid upon a bath of very pure mercury in 
such a way as to form a communication between the nerves and 
muscles, convulsions are usually exhibited. Volta answered 
that, even in this case, there might be some heterogeneity in diffe- 
rent parts of the conducting arc, either upon the surface of the 
mercury or by the contact of the metals, used in preparing the 
animal. Indeed the smallest difference in the substances em- 
ployed to form the communication is sufficient to cause convul- 
sions, when they do not take place without this difference. For 
example, if we arm the nerves of a frog with a sheet of impure 
lead, such as is made use of by glaziers, and then complete 
the communication by an arc of the same metal, taken from the 
same leaf, and consequently of an exactly similar nature, effects 
are rarely produced. But if we complete the communication 
with purified lead such as assayers use, the armature remaining 
the same, convulsions will immediately take place; and it is only 
necessary to rub the arc of a single metal with another metal in 
order to make it sufficiently heterogeneous, as has been shown 
by M. Halle. Nevertheless, Galvani did not yield to these ar- 
guments ; he carried his precautions so far as to prepare the 
organs of the frog with plates of glass, wrought into the shape of 
a knife. He still obtained convulsions with an arc of a single 
metal, but only in the Case which we have mentioned, that is, 
when the animal is very fresh, and in an extremely irritable 
state. Finally, after having prepared the frog with all this care, 
he succeeded in producing the contractions by the mere contact 
of the muscles and nerves of the animal itself, without em- 



Developement of Electricity by simple Contact. 131 

ploying any intervening substances whatever.* But if, as Volta 
affirmed in reply, and as we shall presently prove, electricity is 
developed by the mere contact of two metals, it is equally pos- 
sible that it may be developed by the contact of any two het- 
erogeneous substances, as the muscles and nerves. And if 
this action is much more feeble than that of one metal upon 
another, it will be necessary, in order to detect it, to employ a 
still more sensible electroscope, such as the organs of the frog 
appear to be immediately after death. This new fact observed 
by Galvani serves, therefore, to generalize the idea of Volta, in- 
stead of overthrowing it. 

118. It became necessary, however, to settle the question 
by actual experiment. For this purpose Volta made use of two 
metallic discs, one of zinc and the other of copper, about two 
inches in diameter, the plane surfaces being very true and un- 
varnished, and provided with nonconducting handles for the pur- 
pose of bringing them together and separating them without touch- 
ing them immediately. These discs being brought toward each 
other till they touch, are then to be separated and withdrawn 
in a parallel direction; and their electric state tried by means of Fl S- 52 - 
a straw or gold leaf electroscope. But as the electricity which 
is developed by a single contact is always extremely feeble, we 
do not try directly its repulsive force upon the electroscope ; but 
we previously arm this electroscope with a condenser; we then 
accumulate upon it the electricity of several contacts, by making 
its upper plate communicate with the ground, and touching the 
lower plate with the metallic disc whose electricity we wish 

* To produce this effect, it is necessary to prepare the frog very 
quickly, in the way above described ; then taking it in one hand by 
one of its feet, we hold it in an inverted position, so that the lumbar 
nerves hang down, stretched by the weight of the small fragment 
of the spine which remains attached to them. Then, taking in 
the other hand the other foot, we twist it in such a manner as to 
bring the thigh in contact with the lumbar nerves. If the frog is 
very irritable, the convulsions are immediately produced ; neverthe- 
less, it is sometimes necessary to make trial of several before we 
succeed. This experiment has been contested, but the result is very 
certain if the precautions above given are observed. 



132 Electricity* 

to try, this lower plate communicating with the straws. We 
then withdraw the disc, touch it as well as the other to restore 
them both to their natural state, and place them again in contact; 
we then separate them and bring again to the condenser the 
one we wish to try. After seven or eight contacts of this sort, if 
we remove the upper plate of the condenser, the straws will di- 
verge very sensibly in virtue of the electricity imparted to the 
lower plate, by the successive contacts of the metallic disc ; 
and we then determine the nature of this electricity. That 
the experiment may succeed, it is necessary that the plates 
of the condenser should be without varnish upon the sur- 
faces on which the electricity is deposited ; for in this state 
of weakness, the smallest obstacle would be sufficient to prevent 
its introduction. It is necessary, moreover, for the sake of ac- 
curacy, that each plate should be made of the same metal as the 
disc w 7 ith which it is touched ; otherwise the electric influence 
of this new contact would combine with the effect of the first and 
modify the result. Nevertheless, when it is impossible to com- 
ply with this condition, we may effect the same purpose, by 
placing upon the plates, at the point where they are touched, a 
small strip of paper moistened with water or any liquid that con- 
ducts electricity. For, as we shall presently see, the contact of 
the paper, moistened with such liquid, does not exert upon the 
metals any sensible electrical influence. Let us suppose these 
precautions to have been taken, and for the sake of distinctness 
that the plates are one of copper and the other of zine. If it is 
the disc of copper which has been made to touch the lower plate 
of the electroscope, the electricity which causes the straws to 
diverge will be resinous ; if, on the other hand, the zinc has 
been used, the electricity will be vitreous. Thus these two 
metals, being insulated in their natural state, are brought by 
simple contact into different electrical states ; the copper ac- 
quiring an excess of resinous electricity, and the zinc a cor- 
responding excess of vitreous electricity. 

The experiment may" be repeated in different ways. Let 
neither of the plates of the condenser communicate with the 
ground ; let them both be insulated upon the electroscope, but 
each time that the two discs are separated, touch each of the 
plates at the same time with that disc which is of the same met- 



Dei xlopement of Electricity by simple Contact. 133 

al with itself. As the free electricities which they receive are 
of different kinds, they will mutually attract each other, and 
become fixed upon the contiguous surfaces of the plates. After 
several contacts of this sort, separate the plates, and each of 
them will be found to be charged with the electricity of the disc 
which was made to touch it. 

119. It might be imagined that the electricity which is devel- 
oped in this case depends on a compression of the discs, the one 
against the other, like that which generally developes itself by 
the compression of heterogeneous substances. But it is easy to 
prove that the action occasioned by the contact of metals is 
quite different, and is excited by a reciprocal influence which 
decomposes their natural electricities. To establish this impor- 
tant fact Volta made the following experiment. He formed a 
metallic plate with two pieces Z, C, the one of zinc, the other of pjg, 53, 
copper, soldered end to end. Then, taking between the fingers the 
extremity, of the zinc end, he touched with the other extremity, of 
copper, the upper plate of the condenser, also of copper, the lower 
plate communicating with the ground. After the contact, if the 
plate touched be removed, it is found to be resinously electrified. 
This phenomenon is perfectly conformable to the preceding ex- 
periment; it is to be observed, however, that we have no longer 
to fear the effect of pressure or separation between the particles 
of zinc and those of copper, since their juxtaposition is perma- 
nently established, and since the contact with the condenser 
takes place between copper and copper, and therefore can de- 
velope no new electricity. In order that the electricity thus 
produced by a single contact should be very sensible, it is neces- 
sary that the condenser used should be much larger than that 
of the electroscope and of considerable condensing force. We 
also obtain similar results without touching the zinc plate with 
the fingers, it being held merely by glass rods or any other 
nonconducting substance. But in this case, since the plate in 
question no longer communicates with the ground, it is necessa- 
ry that it should be placed in contact with some body of a large 
capacity, from which it may draw the electricity which it 
is to furnish to the collector plate of the condenser. This is 
done either by giving a larger surface to the zinc plate, or which 
is better, by connecting it with the interior of a large Leyden 



134 Electricity. 

jar, armed within by a plate of zinc, the exterior surface of 
which, being also armed with some metal, is placed in commu- 
nication with the ground. 

This experiment being made, we repeat it in an inverted 
manner. We take between the fingers the extremity of the 
copper part, and with the zinc end we touch the upper plate 
of the condenser which is also of copper. When the contact is 
Fig. 54. destroyed, and we remove the plate touched, it will be found to 
have acquired no electricity, although the lower plate commu- 
nicates with the common reservoir. Nevertheless, the copper and 
zinc still communicate with each other, and touch each other, 
in this case, as in the preceding. The only difference is, that 
in the first case, the two pieces of copper which communicate 
with the zinc are situated consecutively ; while in the second 
experiment, they are situated on opposite sides of the zinc. 
This opposition is therefore sufficient to prevent the condenser 
from becoming charged. Hence Volta concluded that the un- 
known cause which developes the electricity, in the case of zinc 
and copper being in contact, acts like a moveable attractive or 
repulsive force, which is exerted from the zinc to the copper, 
and from the copper to the zinc. Accordingly, in the first ex- 
periment, where the two pieces of copper were on the same side 
of the zinc, this force can exert itself, and the electricity which 
it developes be diffused upon the copper plate of the condenser. 
But in the second experiment, where the zinc is situated between 
two pieces of copper, the electromotive action, whatever be its 
nature, exerts itself equally on the two opposite sides of the zinc ; 
and hence electricity ought not to be developed. 

This explanation agrees with the general circumstances of 
the phenomenon ; but it is not the exact expression of these cir- 
cumstances, still less a necessary deduction from them. All 
which the experiment of Volta shows is, that the zinc and cop- 
per manifest, in the state of contact, a property similar to that 
which heterogeneous bodies present generally, when rubbed one 
against the other. The two electricities cannot remain in equili- 
brium, upon the bodies, in those relations which constitute their 
natural state. If the two metals are insulated, it is necessary 
that the zinc should have a certain excess of vitreous electricity, 
and the copper the corresponding excess of resinous electricity. 



Developement of Electricity by simple Contact. 135 

In this case, by a natural consequence, when the copper is made 
to communicate with the ground, the zinc remaining insulated, 
the same thing takes place which we observe when the rubbers 
of the electrical machine are made to communicate with the 
ground, the glass plate remaining insulated. The resinous elec- 
tricity which exists in excess in the copper, is lost in the ground, 
and the zinc acquires a new excess of vitreous electricity which 
completes its state of equilibrium. The total excess hence re- 
sulting not only diffuses itself over the whole surface of the zinc, 
but is indeed propogated over all the conducting bodies, the na- 
ture of which is such, that in touching the zinc, they do not dis- 
turb the equilibrium of its natural electricities ; in the same way 
as the excess of vitreous electricity, developed by friction upon 
the glass plate of the electrical machine, diffuses itself over the 
neighbouring conductors. And the quantity thus transmitted by 
communication is also limited in the same way, in each case, by 
the condition, that the repulsive force exerted by the charges of 
the two bodies upon the point where they communicate should 
be equal to each other. Now let us place the zinc Z between 
two pieces of copper C, C', one of which C communicates with the ,,. ,. 
ground. Then the relations of the zinc with this piece will be 
such as we have just described ; that is, the piece of copper C 
will be in the natural state, and the zinc Z will have an excess 
of vitreous electricity which may be represented hy -f e. Now 
if this same zinc be in contact by its other face alone, with the 
other piece of copper C, and this communicates directly with 
the ground, the charge of the zinc would still be the same, and 
be represented by -1- e, while that of the copper O would be 
nothing, on account of the communication with the ground. But 
this communication actually takes place through the piece of 
zinc and the lower piece of copper ; and therefore nothing pre- 
vents the piece of copper C from drawing from the ground, 
through these other pieces, the vitreous electricity necessary to 
neutralize the excess of resinous electricity which it has receiv- 
ed from its contact with the zinc, and thus reducing itself to the 
state required by the electric equilibrium, when the copper com- 
municating with the ground is in contact with the zinc. We 
have seen by the first experiment, that this is the natural state 
of the copper when the zinc has -f e ; it will therefore be the 



136 Electricity. 

state of the upper piece of copper C in the present case ; and 
thus the electric equilibrium of the entire system will be express- 
ed in the following manner, where the sign -f designates an ex- 
cess of vitreous electricity. 
The upper piece of copper C', lying upon the zinc, ... 

The piece of zinc Z, below C and upon C, -J- e 

The lower piece of copper C, communicating with the ground, 
We see, therefore, by this analysis of the phenomenon, that 
such a system can in fact communicate no excess of electricity to 
the condenser, when it is held in the hand by the part C, and 
we touch with the part O the collector plate of the condenser, 
supposed to be also of copper. To obtain this result we need 
no Irypothesis ; it is only necessary to express and to apply with 
precision the conditions of the electric equilibrium, given by the 
first experiment, where the upper piece of zinc is alone in con- 
tact with the copper which is below and which communicates 
with the ground. We shall see hereafter that a similar solution 
may be applied with equal facility to many other cases in which 
an electric action takes place, although the zinc is in more or 
less perfect contact with two pieces of metal of the same kind. 
Thus these phenomena, which have been sometimes represented 
as opposed to the theory of the developement of electricity by 
the contact of heterogeneous substances, become in fact so many 
proofs of it, when they are properly considered. 

According to this manner of viewing the subject, when we 
touch the condenser with the copper end of a copper and zinc 
plate, the zinc end of which communicates with the ground, as 
in figure 53, the charge of electricity which the collector plate 
acquires, will not depend on the extent of the surfaces of copper 
and zinc in contact, but only on the repulsive force which 
the electricity exerts when it is in equilibrium at these surfaces ; 
and hence all the plates, whether great or small, must communi- 
cate to the same condenser equal quantities of electricity. This 
is in fact confirmed by experiment. Nevertheless, this equality 
will take place only between the total and final charges ; for as 
to the progress of the charge, it ought to be more rapid with 
large surfaces in contact than with small ones. But the exces- 
sive velocity of circulation of the electricity through the metals, 
renders this difference insensible, for the charge of the conden- 



Developemmt of Electricity by simple Contact. 137 

ser always takes place in so short a time as to appear absolutely 
instantaneous. It may be that sufficient pains have not been 
taken to discover very small differences. Perhaps the form of 
the surface in contact, if it were very small, would affect the 
conditions of the equilibrium, by giving to the electric stra- 
tum, a configuration fitted to render its repulsive force much 
stronger or much feebler ; and this would augment or diminish 
the absolute quantity of the two electricities which might be 
maintained in a state of separation. It is at least a question 
which it would be worth our while to examine ; and it would be 
easy to do it by diminishing the copper and zinc plates at their 
points of junction till they are reduced to simple threads ; or by 
giving to them the form of two convex surfaces touching each 
other at a single point. 

120. The metals, and a great number of substances not me- 
tallic, act upon the natural electricities of each other, like zinc 
and copper, when they are placed in contact ; and it is extreme- 
ly probable that this property extends in different degrees 
to all bodies in nature. Among the various combinations 
that may be formed there are some in which the develope- 
ment is the most active, and others where it is the most feeble 
or even insensible. In the first class are the heterogeneous 
metals, when placed in contact with each other; in the latter 
are pure water, saline solutions, and even the acid liquors, when 
placed in contact either with one another, or with metals. 

To verify this property, take a glass tube open at its two F «g. 56, 
extremities ; close one of them with a stopper of copper termi- 
nating at the bottom in a rod of the same metal and extending 
without the tube. Then fill the tube with one of the liquids of 
which we have spoken, for instance, with water or a saline solu- 
tion or even an acid ; we thus have an assemblage exactly like 
that of the plates of zinc and copper soldered end to end. But 
the electromotive power will be incomparably more feeble. 
For if we try it in the same way, by touching the liquid of 
the tube with the finger, and conveying the rod of copper to 
the plate of the condenser, which is precisely the mode pursued 
in the first experiment, however often we repeat the contact, the 
plate touched will never receive an appreciable quantity of 
electricity; and the same thing would happen, although the li- 
E. &M. 18 



138 Electricity. 

quid contained in the tube should have a very great chemical 
action upon the stopper ; unless we employ very large masses 
of the liquid and metal acting violently upon each other, for 
instance, several pounds of sulphuric acid and iron filings. For 
MM. Lavoisier and La Pierce have observed that, in this case, 
sufficient electricity is developed to charge a condenser so 
highly as to give sparks. The disengagement of this elec- 
tricity results either from the act of chemical combination, 
or from the simple friction of the particles in effervescence 
against each other, and against the sides of the vessel ; it is not 
easy to decide which. But it is evident that the developement 
of electricity which takes place is a different phenomenon from 
that produced by the contact of metals, or other heterogeneous 
substances ; for, in the latter case, the smallest quantities of 
these substances being soldered together, although they produce 
no sensible alteration in each other, yet they communicate to 
the condenser as much electricity as the largest masses. In 
order to prove this distinction with the highest degree of ev- 
idence, we have only to repeat the two operations with masses 
of the same order, which are alternately metals, or one a metal 
and the other a liquid ; for we shall find that the effect of this 
last arrangement is, in comparison with the other, absolutely 
inappreciable. 

121. But, for this very reason, the liquids may serve to 
transmit the reciprocal action of the copper and zinc, without 
Fig. 54 weakening it by their contact. Thus, for instance, returning 
to the second experiment, where the zinc was between two cop- 
per plates, one of which communicated with the ground, we have 
seen that by virtue of the principles of electric equilibrium, 
which belong to such a system, that the two pieces of copper 
must be in their natural state, and the condenser will not be 
charged. But it will charge itself, if, between the zinc and the 
collector plate, which is of copper, we interpose a stratum of 
some conducting liquid, for instance, a drop of water or a piece 
of paper moistened with some saline solution. In fact, since this 
intermediate body may remain in contact with the zinc and 
copper plate, without producing any developement of their natu- 
ral electricities, it follows that it serves only as a conductor from 
one to the other, at the same time that it prevents their immedi- 



Developement of Electricity by simple Contact. 139 

ate contact by its interposition. Thus, supposing the first piece 
of copper to communicate with the ground, the conditions of the 
electric equilibrium of the system will be the same as if the zinc 
were insulated in the air, that is, the zinc will have the same 
excess of vitreous electricity + e which it would have had in 
this case. But besides, as it is at its other surface in contact 
with a system of conductors in which its excess of electricity 
may freely diffuse itself, it will in fact be propogated through 
them ; and thus the condenser will be charged until the quantity 
of electricity, which is not disguised in the collector plate, 
produces an equilibrium by its repulsive force, with the electrici- 
ty -f e of the zinc. And this is in fact perfectly confirmed by 
experiment. 

Consequently, if we make two circular plates to adhere to- 
gether by a strong pressure, one being of zinc and the other of 
copper, and if, after having placed them upon the hand with 
the copper side downward, we cover the zinc face with a moist 
conductor, the contact of which does not disturb the proper 
electric state of the pair, with a piece of cloth, for instance, satu- 
rated with water or a saline solution, any conducting bodies 
which may be placed above this S3 7 stem will partake of the ex- 
cess of vitreous electricity belonging to the zinc face and 
the moist body which covers it. If, therefore, upon this first 
system, we place another of the same kind, in such a way that 
its copper face shall rest upon the moist cloth, this second sys- 
tem will, as a conducting bocly, partake of the excess of vitreous 
electricity belonging to the first zinc surface ; and moreover, the 
second piece of zinc will take a new excess of electricity, also 
vitreous on account of its contact with the copper to which it 
adheres. Adding thus successively several similar systems to 
each other, we shall have an apparatus in which the electric 
state of the successive pieces will go on augmenting from the 
bottom upwards, according to the number of pairs which are 
placed upon each other. 

Such is the admirable instrument universally known at pres- 
ent under the name of the Voltaic Pile or Voltaic Apparatus, from 
which the most surprising results have been obtained in Natu- 
ral Philosophy and Chemistry. To understand its effects, it is 
necessary carefully to analyze the electric state of the different 



140 Electricity. 

pieces which compose it, as well as the changes that take 
place when any of the plates are put in communication with the 
ground or with a condenser. 



Theory of the Voltaic Apparatus, on the Supposition that its conduct- 
ing Power is perfect, 

122. Let us consider in the first place a single pair consisting 
of a zinc plate adhering to a copper one of the same dimensions, 
and let us make the copper face communicate with the ground. 
This face will then be in its natural state, but the zinc face will 
be covered with a stratum of free vitreous electricity, the whole 
quantity of which may be represented by -f- 1. The value of 
this unit will depend upon the extent of the two plates, and it 
will be proportional to this extent. 

The copper face always communicating with the ground, we 
place upon the zinc face a disc of cloth saturated with a saline 
solution, or with any other conducting liquid, which simply di- 
vides by its contact the electricity of the body with which it is 
connected. Then the free electricity of the zinc face will 
diffuse itself over the surface of this conductor ; but as it is al- 
ways necessary that the zinc should possess the excess of vitre- 
ous electricity which its contact with the copper requires, it will 
again take it from the copper, and that from the ground. All 
this is a simple recapitulation of the experiments related in the 
preceeding section. 
Fig. 57. Things being in this state, we take a new pair of copper and 
zinc plates similar to the first $ and, after having touched the 
copper face, we insulate it, and place this face upon the 
moist cloth. Then, according to the theory of Volta, two ac- 
tions take place ; (1.) The zinc face of this second piece pre- 
serves the excess of vitreous electricity -f 1, derived from its 
contact with the copper ; (2.) The whole piece partakes of the 
free electricity of the cloth, as any other conducting body would 
do. The cloth disc takes this electricity again from the lower 
zinc plate, that from the copper, and the copper from the ground ; 
and thus after an infinitely small interval of time, if the conduct- 



Theory of the Voltaic Apparatus. 141 

ing power be perfect, a stable electric state is established, in 
which the quantities of free electricity are such as are repre- 
sented in the following table. 

C Zinc face z 2 adhering to c 2 . . . . +2 
The upper pair < Copper face c 2 communicating with 

( the moist cloth +1 

CZinc face z, adhering to c, . . . . +1 
The lower pair < Copper face c, communicating with 

( the ground 

Upon this system place a second circle of cloth, and then a Fig. 58. 
third pair of copper and zinc plates. The zinc piece of this 
new pair will preserve the excess of vitreous electricity -}- 1, de- 
rived from its contact with the copper ; but besides this, it will 
partake, as a conducting body, of the free electricity of the 
lower pieces which will be replenished from the ground ; and 
when these electric states have become stable, we shall have them 
represented in the following table ; 

CThe zinc face z 3 adhering to c 3 -f- 3 

Pair 3d. < The copper face c 3 communicating with the 

( moist cloth m 2 -\~ 2 

C Face z 2 adhering to c 2 -}- 2 

Pair 2d. < The copper face c 2 communicating with the 

( moist cloth w, -{- 1 

CZinc face z, adhering to «, -|- 1 

Pair 1st. < Copper face c, communicating with the 

( ground 

Continuing thus to add other pairs of plates, the quantities of 
free vitreous electricity will increase from the bottom upward, 
according to an arithmetical progression. 

1 23. This theory supposes that the transmission of the elec- 
tricity takes place through the moistened cloths without any 
diminution ; which is the case when the conducting power is per- 
fect. It is supposed, moreover, that the liquids interposed be- 
tween the pairs of metallic plates, are absolutely electrified only 
by communication, or at least if their contact affects the free 
distribution of the electricity, the change which they occasion 
is so weak that it may be neglected. Finally, in the passage 
from one element to another, a third supposition is made ; and this 
is that the excess of electricity -j- 1 5 which the zinc takes from 



142 Electricity. 

the copper, is constant for these two metals, whether they are in 
their natural state or not. This last supposition is the most sim- 
ple that can be made ; but nevertheless it is only a supposition 
of which the fundamental experiments related above, furnish no 
proof. Coulomb undertook to verify this law, and it appeared 
to him to be exact. It is evident that it can be established with 
precision only by the aid of the electric balance, and by 
measuring the quantities of free electricity at different heights 
of the pile ; but the results are affected by the imperfect con- 
ducting power of the moistened conductors, and by several 
other causes which will be examined in the following sections. 
Let us admit, for the present, the progression in question, as the 
most simple law which we can imagine, and let us endeavour to 
develope the consequences which result from it by calculation. 
In the first place, if we touch the base of the pile with one 
hand, and place the other hand on the top, all the excesses of 
electricity -{- 1, -f- 2, -f- 3, &c, of the different pieces will be 
discharged through the organs of the body into the common 
reservoir. Supposing the transmission of the electricity in the 
interior of the pile perfectly free, or merely very rapid compar- 
ed with its transmission through the organs, this discharge will 
produce a shock like that of the Leyden jar, but with this re- 
markable difference, that the sensation will appear to be contin- 
uous ; for, the pile recharging itself from the ground much more 
quickly than the organs can discharge it, the upper piece will con- 
stantly be found to be almost as highly charged as before the con- 
tact. Experiment perfectly confirms these inferences. We may 
also repeat the experiments in which the taste and sight are 
affected, only the effect will be much greater than in the case of 
a single pair of pieces. 

124. If we wish to determine in this case the quantity of 
electricity which forms the discharge at each contact, we have 
only to take the sum of the quantities of electricity which, 
according to the preceding inferences, exist in a state of free- 
dom in different parts of the apparatus. But, in order to sim- 
plify this determination, we may suppose the moist cloths to be 
infinitely thin, and we may neglect the quantity of electricity 
which tends to their exterior surface ; then the preceding quan- 
tities spread over the surfaces of the copper and zinc, will be 



Theory of the Voltaic Apparatus. 143 

the only ones which it is necessary to consider. In this way 
we find that their sum is proportional to the square of the num- 
ber of pairs. But it will be seen hereafter that the imperfection 
of the moist conductors very much diminishes the result. 

125. We have supposed the pile composed in the following 
way; copper, zinc, moist cloth, copper, and so on, the first 
piece of copper communicating with the ground. But we might 
also proceed in the contrary order, zinc, copper, moisture, zinc, 
establishing a communication between the ground and the first 
piece of zinc. In this case, the theory will be absolutely the 
same, with this single difference, that our unit + 1 would become 
negative, that is, the quantities of free electricity would be of 
the resinous kind. 

126. Instead of placing the metallic plates upon each other Fi S- 59 « 
in a vertical column, they may be placed horizontally, and par- 
allel to each other upon insulating supports, for instance, upon 

rods of varnished glass. Then, instead of interposing between 
them pieces of cloth, cells may be formed from one to the other to 
receive the liquid which is to serve as a conductor ; this arrange- 
ment is called the Galvanic or Voltaic Trough. We may also sol- 
der together, end to end, plates of copper and zinc inclining them 
a little at the soldered point, in such a way that each metal may Fig. 60. 
be immersed in a glass or porcelain vessel, partly filled with the 
conducting liquid. A series of such vessels forms an electromo- 
tive chain, the extremities of which may be made to meet for 
the convenience of experiments. This Volta called a crown of 
cups. We shall hereafter examine the inconveniences and ad- 
vantages peculiar to each of these constructions ; it is sufficient 
here to indicate the different arrangements. As to the mode of 
electric action, it is exactly the same in all, and the theory 
which we have explained, is equally applicable to each. 

127. Let us now apply to the upper part of the pile, or in 
general to the last plate of the apparatus, a condenser, the lower 
plate of which communicates with the ground. Before the con- 
tact, this plate, which I suppose always to be of zinc, possessed 
the free vitreous electricity belonging to its place in the pile. 
The condenser takes from it a part of its electricity which is 
immediately supplied from the lower piece, that from the next 
following, and so on to the last, which takes the whole from the 



1 44 Electricity, 

ground. This motion ought therefore to continue until the up- 
per piece has resumed the same quantity of free electricity 
which it at first possessed, and which belongs to its place or 
rank. Thus the condenser will charge itself until its collector 
plate have the same intensity as this piece* 

If the pile were formed in a contrary order, the zinc com- 
municating with the ground, the free electricity at its summit 
would be resinous, and the charge of the condenser would be 
equal to what it was before, but resinous. 

As the electricity of the Voltaic apparatus accumulates in the 
condenser, it will accumulate in the interior of a Leyden jar or 
of an electric battery, the exterior coating of which communi- 
cates with the earth ; and since, according as the pile is discharg- 
ed, it recharges itself from this common reservoir, the latter will 
also charge itself, whatever be its capacity, until the repulsive 
force of its free electricity is in equilibrium with that which ex- 
ists at the top of the pile. Then if the battery be withdrawn 
from the pile, it will give a shock corresponding to the degree 
of repulsive force. And this is confirmed by actual experi- 
ment. 

128. In order that the action of the condenser upon the pile 
may be as regular, constant, and powerful as possible, it is 
necessary that the greatest care should be taken to establish a 
perfect communication between its coating and the poles of the 
pile ; for the quantities of free electricity being exceedingly 
small, the least obstacle is sufficient to arrest or obstruct materi- 
ally the transfer ; and in this case the condenser receives much 
less electricity than it would do if the communications were free. 
It is still worse if the medium of communication is itself variable, 
as when we hold the condenser in the hand, and content ourselves 
with placing the knob of the collector plate upon the summit of 
the pile. In this case, if we apply it several times in succession 
to the same pile, the quantites of electricity with which it becomes 
charged, may sometimes be three or four times as great as at 
others ; whereas with a more uniform medium of communication, 
we should find a perfect equality. Now this uniform medium is 
absolutely necessary in order to determine exactly the measure 
of the electric state of the pile. 



Theory of the Voltaic Apparatus. 145 

After many trials, I have thought the following arrangement 
the most convenient. Upon a solid table, I fix by means of 
screws a parallelopiped of wood A-B, covered with a plate of 
tin. The extremity A of this parallelopiped supports a metal 
cone, truncated at the top and well polished, upon which we °' 
place the pile ; the other extremity B carries a vertical and 
moveable metallic rod TT, terminated by a metallic plate to 
which the foot of the condenser is firmly fixed by a metallic 
screw. We may thus bring this instrument to the height of the 
pile subjected to the experiment, without altering the exactness 
of the communications. The discs of which 1 make use are all 
of the same dimensions, and each disc of zinc is forcibly pressed 
and made to adhere by a rim to the corresponding disc of cop- 
per. In this way, the contact is always perfectly established 
between them. We have only to dispose of a certain number 
of pairs, one above the other, and these pairs may be considered 
as perfectly similar when the plates are new; as they are more- 
over very regular, it is sufficient, in making the pile, to place 
them one upon the other without lateral supports, by which 
method we avoid the kind of communication which takes place 
between the poles of the pile by the imperfect insulation of these 
supports, to the great injury of the apparatus. 

Finally, in order to establish constantly and uniformly, the 
contact of the condenser with the summit of the pile, I place up- 
on the pile a small iron vessel filled with mercury, and made 
very clean at the bottom ; the knob of the condenser and the 
extremity of its flexible rod are also of iron. In this way, when 
the instrument is brought to the height of the pile, it is sufficient 
to depress its knob into the mercury by means of a tube of var- 
nished glass ; after which, leaving the rod to its own elasticity, 
we are certain of having as equal and instantaneous a contact as 
is possible. We may also, if we choose, continue it for a longer 
time, in order to determine if the time has any influence 
on the charge of the condenser. When the rod has emerg- 
ed from the mercury, we remove the collector plate parallel to 
itself, and touch it with the fixed and insulated ball of the s. 
electrical balance. This we return into its case of glass ; the 
moveable disc, which I suppose in the natural state, touches it, 
and is immediately repelled to a certain distanceewhich we ob- 
E. &M. '19 



146 Electricity. 

serve ; or, if we please, we twist the suspending wire, until the 
disc is brought to a fixed distance from the ball. Whichever 
of these methods we adopt, as the disc will be electrified by the 
contact, and at the expense of the ball, the angle of torsion will 
measure the square of the quantity of electricity communi- 
cated to the ball by the condenser, and to the latter by the 
pile. In this way we can estimate this quantity. I have ascer- 
tained that by making use of this method, we obtain by a series 
of experiments, results capable of being accurately compared ; 
which is very far from being the case, when we neglect the pre- 
cautions above given. 

129. Comparing in this way the charges obtained with piles 
of the same number of plates connected by moistened conductors 
of different kinds, we find that water, weak acids, moist saline 
solutions, and generally substances of a high conducting power, 
give the same sensible quantity of free electricity, and give it 
by a contact apparently instantaneous. Indeed, for bodies of 
the greatest conducting power, we may very much increase or 
diminish the extent of surface, without any apparent variation 
of the charge, undoubtedly on account of the extreme facility 
with which the surface transmits the electric current; and this 
is sufficient to prove, agreeably to the opinion of Volta, that they 
perform the office of conductors merely, and that their contact, 
or their chemical action, is not the determining cause of the de- 
velopement of electricity. Nevertheless, we also find liquids 
with which the charges are unequal, for the same number of pairs, 
either because they too much weaken the conducting power by 
their interposition, as we shall show hereafter, or because they 
modify the conditions of the electric equilibrium by their contact, 
or by the nature of the combinations which they form with the 
other parts of the apparatus. All these varieties have presented 
themselves in the numerous experiments made by philosophers 
since the time of the first use of the instrument. 

130. In what precedes, we have supposed that the electro- 
motive apparatus communicated by its base with the ground, 
and thence received all the free electricity necessary to the 
equilibrium of its parts. But if we imagine that all the pieces 
which compose it are placed originally upon an insulater, and 
that the apparatus itself and the person who constructs it are 



Theory of the Voltaic Apparatus. 147 

insulated during the operation, then the quantities of free elec- 
tricity, necessary to an equilibrium, being prevented from com- 
ing to the ground, the pile would derive them from itself by the 
decomposition of the natural electricities of its plates. The zinc 
pole would therefore have an excess of free vitreous electricity, 
compensated by an equal excess of resinous electricity at the 
copper pole ; and proceeding thence, the quantities of free elec- 
tricity would go on decreasing to the middle of the column, 
which would be in the neutral state. In fact, it is evident that 
in this way the conditions of equidifference from one piece to 
another would be satisfied, and would preserve the order which 
we have assigned them in the uninsulated apparatus. These 
views are confirmed by experiment, at least in their general re- 
sults, for all piles, even after having been connected while 
in communication with the ground, come of themselves to the 
state which we have described when they are placed for some 
time upon an insulater, because the air which touches them 
gradually taking away their free electricity, they cannot become 
recharged except from themselves ; and the results of this decom- 
position are all which remain, when the quantities of electricity 
which they had originally drawn from the ground have at length 
been wasted. In this state, the signs of electricity at the two 
poles of the pile are very feeble ; and the best condensers are 
not sensibly charged by them. This phenomenon is the more 
worthy of notice, since it does not accord with the theory of 
equilibrium by equidifference. This theory, indeed, indicates 
that the charge of the condenser from the insulated pile must 
be less than that from the pile which is not insulated ; but the 
proportion which it gives is very far from that extreme weakness 
indicated by experiment. 

In reflecting on this disagreement, I have been led to think 
that the electric action of the electromotive apparatus might not 
be owing simply to the quantities of free electricity which appear 
upon its elements, as Volta supposed ; but that there might exist 
there at the same time a very great quantity of disguised elec- 
tricity ; and as this consideration places the action of the pile 
in a new point of view, I shall here explain myself. 

131. Let us first recur to the fundamental experiments of 
Volta on the developement of electricity by the simple contact 



MB Electricity. 

of two insulated metals. We learn that there is manifested upon 
the two metals a certain quantity of free electricity of opposite 
kinds. But it does not therefore follow that these quantities are 
the only ones which are really developed by the contact ; and 
the decomposition of the natural electricities of the two plates, 
during the contact, might be indefinitely great without producing 
any other external indications than those which we have observ- 
ed. It is in this way that two faces of a pane of glass armed 
with metal may be charged with very considerable quantities of 
electricity, although the portions of these electricities which ex- 
ert their repulsive force are extremely small. 

Considered in this way, two discs of zinc and copper being 
placed in contact would exactly resemble such a pane of glass, 
after it has been insulated, and when the absorbing action of the 
air has equalized the repulsive forces. Only for the noncon- 
ducting plate of glass would be substituted the unknown forces 
which retain the two electricities on one side and the other of 
the surfaces in contact. Then the electroscope and the electric 
balance would render sensible only those portions of electricity 
which were free ; and the whole quantity of disguised electrici- 
ty would be manifested only at the instant when a direct com- 
munication was established between the discs, as in the Leyden 
jar or the electrified pane of glass. 

The Voltaic apparatus would thus become altogether anala- 
£3, fcc.gous to the electric pile above considered. The disposition of 
the electricity would be exactly the same, and the same theory, 
and the same formula, would apply to it. We may, in fact, re- 
mark, that the results to which we have arrived in considering 
the pile, offer an exact representation of the electric phenomena 
produced by the Voltaic apparatus, both when one of its poles 
communicates with the ground, and when in a state of insulation. 
Considered in this way, it is more easy to conceive how it is 
capable of giving such, strong shocks, and of producing such 
chemical effects which are ordinarily obtained only by the aid 
of large quantities of electricity, either by batteries, or by 
means of points of an extreme fineness. It is, in fact, because 
there is a very great quantity of electricity developed in the 
chemical action of the electromotive apparatus. Finally, we may 
then conceive why the most powerful piles, when they are insulated 



Chemical Effects of the Voltaic .Apparatus. 149 

at tjieir base, communicate hardly any sensible electricity, while 
they give considerable charges and even sparks if one of their 
poles is made to communicate for an instant with the ground. 
For the charges indicated by calculation for these two cases 
would in fact be extremely different ; whereas they would not 
according to the first mode of viewing the subject. 



Chemical Effects of the Voltaic Apparatus. 

132. The first chemical effect produced by the pile, was the 
decomposition of water. This discovery is due to MM. Carlisle 
and Nicholson. If we adapt to the poles of the electromotive 
apparatus, platina wires leading into a glass vessel partly filled 
with water, we shall see a continued current of oxygen gas dis- 
engaging itself from the wire which communicates with the vit- 
reous pole, and at the same time a current of hydrogen gas dis- 
engages itself from the other wire which communicates with the 
resinous pole. If instead of platina wires, we employ wires of 
copper, silver, or of any other metal which is easily oxidated, 
the hydrogen continues to appear at the resinous wire ; the ox- 
ygen no longer disengages itself under the form of a gas, but com- 
bines with the wire and oxidates it. It is of no importance 
whether the pile be insulated or not. 

To determine whether the two gases which are disengaged 
are really in the proportion which constitutes water, it is neces- 
sary to collect and measure them. The most convenient appara- 
tus for this purpose is represented in figure 62. EE is a glass 
tunnel of which the mouth B is closed by a cork stopper, through 
which are made to pass two small hollow tubes of glass at the 
distance of about one third of an inch from each other, and of 
which the extremities, both within and without, extend a little 
beyond the two surfaces of the cork. Each tube serves as a 
case to a platina wire, which is cemented in it with sealing wax, 
so that the tubes are perfectly closed. The whole is arranged 
in such a way, that the two wires rise parallel to each other, in the 
interior of the tunnel, to the height of one or two inches above 
its bottom. We pour some water into the tunnel, and cover 



150 Electricity. 

each wire with a small glass bell also filled with water. Final- 
ly, we make the external parts of the wires communicate, each 
with one pole of the pile, and the apparatus is complete. We 
suffer it to act for some time, after which we stop the action, and 
measure the volume of gas disengaged under each bell. We 
find twice as much hydrogen as oxygen in bulk. These are in 
fact the proportions which constitute water ; for upon re-establish- 
ing the combination by means of the electric spark, and the small 
94 apparatus above described, no gaseous residue remains. That we 
may lose nothing of the action of the pile, it is necessary that the 
communication of the wires with the extreme plates should be 
perfectly established. For this purpose, there is no method 
more convenient than to immerse them in a small glass vessel 
filled with mercury, in which are also placed two large iron 
wires connected with the extreme plates of the electromotive 
apparatus. 

133. With this apparatus, MM. Gay-Lussac and Thenard 
observed that the quantity of gas disengaged in a given time by 
the same pile, whether constructed with pieces of cloth or in 
the form of troughs, varies considerably, according to the na- 
ture of the substances dissolved in the water with which the tun- 
nel is filled. Concentrated saline solutions and mixtures of 
water and acid give most abundant and most rapid decomposi- 
4ions. The result diminishes according as the proportions of 
salt or acid are less ; and finally, when the tunnel contains only 
boiled and perfectly pure water, scarcely any gas is disengaged. 
It appears that, in this case, the interposition of the water be- 
comes a sufficient obstacle to prevent the circulation of the elec- 
tric current from one pole of the pile to the other ; but if we 
introduce into the arc of communication the most delicate bodily 
organs, all the effects which the voltaic apparatus ordinarily 
produces cease, at least when the communication is established 
through the water itself. Thus pure water, which transmits a 
strong electricitjr, such, for instance, as we obtain from our com- 
mon machines, becomes almost a nonconductor for the feeble, 
repulsive forces furnished by the voltaic apparatus. Hence 
we may here apply the general laws which we have discovered 
relative to imperfect conductors : that is, for a given distance of 

wires, the insulation will be perfect only for a certain degree 



Chemical Effects of the Voltaic Apparatus. 151 

of repulsive force, determined by the number of plates of the 
apparatus ; so also for each nonconducting support, the degree 
of repulsive force, where perfect insulation commences, is as the 
square roots of the lengths of the supports ; so also for each elec- 
tromotive apparatus, there must be a certain distance of the wires 
at which the communication will be entirely interrupted. We 
must find here, in like manner, the influence upon the insulation 
arising from the more or less extended contact taking place be- 
tween the support and the insulated body. Thus MM. Gay-Lussac 
and Thenard have remarked, that by shortening the wires beyond 
a certain limit, the quantities of gas disengaged in the same fluid 
are considerably diminished ; they are increased anew by sub- 
stituting a liquid of a greater conducting power. This want of 
conducting power in the water may be immediately rendered 
sensible by a very simple experiment.. Having insulated a pile 
and placed conducting wires at its two poles, immerse these 
wires in a glass vessel partly filled with common water 5 the 
gases will be immediately disengaged in abundance. If we 
withdraw one of these wires from the water, and taking it in one 
hand, immerse the other hand in the water of the vessel, we 
shall feel a shock as usual. But instead of this, make the com- 
munication by means of a column of water a fifth or a sixth of an 
inch in diameter, and an inch and a half or two inches in length, 
which may be done by drawing up the water of the vessel with a 
tube of these dimensions ; then, although the most delicate organs 
are within the arc of communication, a feeble effect upon the 
taste, but not the slightest convulsion, will be perceived. I have 
in this way arranged a pile of sixty-eight pairs of plates, the 
poles of which communicated by tubes that were not capillary, 
filled with distilled water, and about forty inches in length. 
The apparatus was in action for twenty four hours, without dis- 
engaging a particle of gas ; in endeavouring to make a communi- 
cation from one pole of the pile to the other by means of the 
columns of water contained in the tubes, not the slightest sensa- 
tion was felt, which the electromotive apparatus ordinarily pro- 
duces. In fact, the whole took place as if an insulating body 
had been interposed between the two poles; but all the effects 
reappeared as soon as an immediate communication was made 



152 Electricity. 

by the free surface of the water.f On this account it is to be re- 
gretted that, in the experiments of MM. Gay-Lussac and Then- 
ard with distilled water, they had not attemped to extend the 
wires over the surface of the water likewise ; for I think that in 
this case, the communication between the two poles of the pile 
would have been established. 

134. MM. Gay-Lussac and Thenard endeavoured to ascertain 
whether there be not some ratio between the quantities of gas disen- 
gaged by the pile, and the quantities of salt thrown into the water 
of the tunnel ; but they have no simple relation except for the sul- 
phate of soda. The quantities of gas disengaged in a given time are 
very nearly proportional to the cube root of the quantities of salt 
contained in the water of the tunnel. The solution of nitre pro- 
duces a contrary effect ; being saturated with this salt, water 
yields less gas than when not saturated. But it is necessary to 
observe that the decomposition of the water is not the only phe- 
nomenon which takes place in these experiments. Most of the 
substances which are dissolved in this liquid, and subjected 
with it to the action of the electric current, suffer also changes 
in their constitution. We are not therefore to expect to find a 
constant or simple relation between the absolute energy of the 
apparatus and the mere disengagement of the gases. 

The first example of this action of the pile upon the different 
substances contained in water was observed by Cruikshank 
in repeating the experiment of Nicholson and Carlisle. Having 
employed, as a conducting medium, water charged with acetate 
of lead, he saw that the resinous wire was covered with a mul- 
titude of small needles of metallic lead. He obtained analogous 
effects with solutions of sulphate of copper and of nitrate of sil- 
ver. In the latter, the small needles of silver were articulated 
upon each other, like a species of vegetation, so as to form, by 
their union, what chemists call the tree of Diana. The electric 
current, therefore, disengaged the metals from their combination 
with the acids which held them in solution, and with the oxygen 
which is necessary to dissolve them, in the same way as in the first 
experiment on water alone, it separated the hydrogen from the 
oxygen with which was combined ; and in both cases alike, the 



T Journal de Physique^ an 9. (1800). 



Chemical Effects of the Voltaic Apparatus. 153 

wxygen was developed at the vitreous pole, while the substances 
with which it was saturated, became free at the resinous pole. 

135. In order to study the nature of the power which pro- 
duces these decompositions, Cruikshank caused the voltaic cur- 
rent to pass through solutions charged with blue vegetable col- 
ours, which have the property of turning red by the contact of 
an acid, and green by that of an alkali. He observed that 
the first effect took place about the vitreous wire, and the second 
about the resinous wire. This experiment may be performed 
with ease and elegance in the following way, according to Singer. 
We bend a small tube of glass into the form of the letter V, and 
introduce into each branch a platina wire which is made to 
communicate with the two poles of the voltaic pile. We then pour 
into the tube a solution of red cabbage, which is of a delicate 
blue colour and very sensible to the action of acids and alkalis.* 
The decomposition of the water immediately begins to take 
place as usual, and the two gases which constitute it are disen- 
gaged ; but besides this, after a short time, Ave shall see the li- 
quor become red about the vitreous wire, and green about the 
resinous wire. W^hen this effect has become very evident, in- 
vert the communications of the two wires, by changing the poles 
to which they are applied, and suffer the apparatus to act anew. 
The red will soon disappear from one side, and the green from 



* The following is the method recommended by Mr Singer for pre- 
paring the solution. When we wish to obtain a very sensible re- 
agent, we infuse for a few moments thin leaves of red cabbage in a 
quantity of warm water just sufficient to cover them. This water 
takes a beautiful blue colour, which the contact of acids changes 
to red, and that of alkalis to green. The solution can be preserved 
but for a very short time without undergoing a change. We obtain 
a more durable reagent, but one which is somewhat less delicate, 
by adding several drops of sulphuric acid for every pint of water 
which is employed in forming the infusion. In this case the colour 
of the infusion is red ; and when we wish to make use of it, we take 
a small quantity and neutralize it by the application of a few drops 
of ammonia until the blue colour re-appears ; but the difficulty of 
obtaining the precise point of neutrality must render this prepara- 
tion less sensible than the first. 

E. <&■ M. 20 



1 54 Electricity. 

the other ; the liquor will become blue again in the two branch- 
es ; and after a time, each colour will be found to be replaced 
by its opposite. When these phenomena were first observed, 
it was inferred that the electric power actually formed an acid 
about the vitreous wire, and an alkali about the resinous wire, 
but farther researches, which were principally those of Sir 
Humphrey Davy, have shown that these phenomena were simp- 
ly the result of decompositions produced by the electric current 
in the media through which it is made to pass. This able che- 
mist found that, in order to prevent them, it was necessary to 
employ every possible precaution. For instance, he still 
obtained signs of alkali and acid, when he made the voltaic 
current pass for some time through perfectly pure water, con- 
tained in different glass vessels communicating only by indisso- 
luble fibrous substances, such as films of amianthus, or of asbes- 
tos, saturated with water. In this case, the alkali is obtained 
from a partial decomposition of the glass itself; the acid is form- 
ed by the oxygen disengaged from tlie water, which, being in 
the nascent state, combines with the nitrogen of the surrounding 
atmosphere, and constitutes nitric acid. These traces of nitric 
acid were still sensible, although very weak, when common dis- 
tilled water was used, placed in gold cups ; but it was found also 
that such water was not perfectly freed from every foreign sub- 
stance. Finally, the attempt succeeded by employing water 
distilled very gradually in alembics of silver, and exposing it to 
the electric current in vessels of gold. All traces of alkali and 
acid now entirely disappeared. 

136. This enquiry, while it proved the great power of the 
voltaic apparatus as an instrument of chemical decomposition, 
showed also the necessity of guarding against the effects of 
its action upon the vessels themselves containing the solutions 
which were to be subjected to trial. The experiments, 
which required much exactness, it was necessary to perform in 
cups of gold or agate withdrawn from the contact of the air. 
The solutions which it was proposed to try, were put into diffe- 
rent cups, and a communication was established between them 
by means of films of amianthus. But new and unexpected phe- 
nomena were now presented. Substances, which were at first 
mingled, and distributed uniformly through the conducting me- 



Chemical Effects of the Voltaic Apparatus. 155 

dium, separated under the influence of the voltaic current, and 
each of them was found collected in one cup, apart from the 
other. Others which had been at first placed in different cups, 
were found to have changed places ; so that it was necessary to 
recognise in this current a particular power of transfer, which 
collected in general the acid principles at the vitreous pole, and 
the salifiable bases at the resinous pole. This beautiful discov- 
ery was made by two Swedish chemists, Berzelius and Hisin- 
ger. 

Let us suppose, for instance, that we employ but two cups, 
and that we fill them both with a solution of sulphate of soda. 
After an action of some hours, all the salt is found to be decom- 
posed ; the cup which communicates with the vitreous pole con- 
tains a solution of sulphuric acid, and we find that the soda is in 
the cup which communicates w r ith the resinous pole. It is ne- 
cessary, therefore, for this effect to take place, that the alkali and 
the acid should have entirely passed from one cup to the other 
along the films of amianthus, or rather along the particles of wa- 
ter which moisten these films. 

137. This experiment may be varied by employing three 
communicating cups, of which the two extreme ones contain only 
distilled water charged with the blue infusion of red cabbage, 
while that in the middle contains a solution of sulphate of potash. 
We place the two extreme cups in communication with the two 
poles of a voltaic pile ; after some time we find the salt of the 
middle cup decomposed, and its separate elements transferred 
into the two others. The acid passes to the cup which com- 
municates with the vitreous pole, and reddens the blue liquor 
contained in it, while the alkali goes to the liquor which com- 
municates with the resinous pole, and changes it to gredn. 

138. A very remarkable circumstance in this transfer is, that 
the substances transferred are always carried through media for 
which, in their ordinary state, they have a very strong affinity, 
yet without combining with them permanently in their passage. 
The following is one example among many others of this fact. 
We employ three communicating cups ; the first, in which the 
resinous wire is immersed, contains a solution of sulphate of pot- 
ash ; in the second, we place a solution of ammonia, a substance 
having a very great affinity for sulphuric acid. The third ? in 



156 Electricity. 

which the vitreous wire terminates, contains only pure water. 
When the pile begins to act, the electric current decomposes the 
sulphate, retains the potash in the first cup, and transfers the 
acid into the third, where it is found free, although to arrive 
there, it must have passed through the ammoniacal solution. 
]f instead of the ammonia we substitute an acid, and immerse 
the vitreous wire in the solution of the sulphate of potash, it is 
the potash which is transferred, and it goes into the cup con- 
taining the resinous wire ; and this it does by passing through 
the intermediate acid, without being retained by its affinity for 
that substance. And not only do the products transferred thus 
resist very powerful affinities, but the most sensible reagents 
appear not to he affected by their passage, and give no indica- 
tion of it. Let us suppose, for instance, that we employ, as be- 
fore, three communicating vessels, two of which, namely, that in 
the centre, and that which the vitreous wire enters, contain a 
neutralized infusion of red cabbage, while we put a solution of 
sulphate of potash into the third which receives the resinous 
wire. After having made the vessels to communicate by moist- 
ened films of amianthus, or of cotton, if we cause the voltaic cur- 
rent to act upon the liquors which they contain, the sulphate 
will be decomposed, and the acid will pass into the liquor of the 
vitreous vessel, .which it will redden, without altering in any 
way the colour of the intermediate solution which it must never- 
theless have passed through. If we invert the communications 
of the extreme vessels with the pile, the transferred potash will 
present an analogous effect. It seems, therefore, that the elec- 
tricity attaching itself, as it were, to the particles which it trans- 
fers, modifies the natural affinities, and modifies them differently 
according to their nature. This result is the more surprising, 
since, when we examine the mode of distribution of the electric- 
ity among bodies of a sensible magnitude, we find that it diffuses 
itself over them in proportions depending upon their form simply, 
without manifesting any particular affinity for the matter which 
composes them. But as I have already hinted, it is possible that 
the smallness of the material particles to which the voltaic cur- 
rent attaches itself, may explain this apparent contradiction ; for 
the absolute quantity of electricity, whether vitreous or resinous, 
with which the particles of each substance may become charged 



Chemical Effects of the Voltaic Apparatus, 1 57 

in a given medium, must depend on their configuration and con- 
ducting power; which power, as we shall hereafter see, is some- 
times very different for the two electricities, when the repulsive 
force is reduced to the degree of weakness in which it exists in 
the voltaic current. Thus the mere difference of form and of 
conducting power, may be sufficient to determine the kind and 
the inequality of their electric charges, without its being neces- 
sary to admit the existence of a true affinity between the par- 
ticles of the substances and the principle of the two electrici- 
ties. 

Not being able to observe immediately what takes place in 
the very act of transfer, since the transferred substances are al- 
ways invisible during their passage, it is necessary to seek m 
the definite results of this phenomenon for the conditions which 
limit or modify it, and to employ them as so many characteristic 
indices of the manner according to which it has occurred. Un- 
happily these results are for the most part chemical effects, the 
manner of whose production is equally incapable of being ob- 
served, and which become sensible to us only after they have 
taken place. But although our ignorance respecting the condi- 
tions on which they depend, and the particulars of their produc- 
tion, renders it difficult to employ them as characteristics, and 
is an invincible obstacle to a complete analysis of the electric 
influences which determine them, it is only the more necessary 
at present to carry this analysis as far as the actual data will 
permit ; for it is by separating the certain from the probable and 
the unknown, that we are able to perceive accurately the diffi- 
culties to be removed, the elements which are yet necessary to 
establish a complete theory, and consequently the points to 
which it is necessary to direct our future inquiries in order that 
they may turn to some account. 

139. The first thing to be considered in this analysis, is the 
electric state of the liquid media interposed between the poles 
of the pile, and serving as conductors for the transmission of the 
electricity. Now this state is rendered sensible by an experi- 
ment of Volta, which consists in uniting the poles of the pile by 
a liquid conductor sufficiently imperfect for sensible differences 
of charge to exist between its different parts. For instance, this 
object is perfectly obtained by a long band or strip of paper 



1 oS Electricity, 

saturated with pure water. After the communication has been 
thus established for several instants between the two poles, if we 
touch successively different parts of the paper with an electros- 
cope provided with a condenser, in order to determine its elec- 
tric state, we find that when the pile is insulated, each half is 
charged with a kind of electricity belonging to the pole to which 
it adheres; the one is vitreous, the other resinous; and the in- 
tensity of these charges goes on decreasing from each pole to 
the middle of the band which is in a neutral state, at least if we 
suppose a conducting power constant throughout its whole length - f 
for if we render the passage of the electricity more easy upon 
one of the two halves than upon the other, as may be done by 
applying several drops of saline solution which has a greater 
conducting power than pure water, the electric charges of this 
half become stronger at equal distances, and the neutral point 
approaches the opposite pole. When the pile, instead of being 
insulated, communicates with the ground by one of its poles, the 
neutral point passes to this pole itself, and all the rest of the band, 
with a progressively increasing intensity of charge, presents 
merely the kind of electricity which belongs to the insulated 
pole. 

140. In this case, as in the last, the law according to which 
the progressive diminution of the charges takes place from one 
pole to the other, must depend on the more or less perfect con- 
ducting power of the liquid by which the band is moistened, and 
on the greater or less extension presented by the surface upon 
which it is spread. But the manner according to which the 
passage of the two electricities, and their mutual neutralization 
takes place, must remain the same in its general circumstances ; 
for it is only a simple application of the ordinary law which 
electricity observes, when it distributes itself over imperfect con- 
ductors. It ought, therefore, to be the same, when the two poles 
of the pile, instead of being joined by a single moist lamina, are 
.joined by an infinite number of such laminae, united together in 
.such a way as to form a fluid mass, and charged with quantities 
of electricity sufficiently feeble, compared with their conducting 
power, not to be rapidly communicated from one to another. 
In this case, each of the fluid threads commencing at one of the 
points of the vitreous wire, and terminating at one of the points 



Chemical Effects of the Voltaic Apparatus. 159 

of the resinous wire, may be considered as an imperfect conduc- 
tor, to which all that we have said respecting the electric state 
of the band is applicable, except the modifications required in 
the more or less rapid diminution of the intensities ; that is, the 
passage of either electricity must take place along each of these 
with differences of velocity and charge depending on their 
length, on their conducting power which may be unequal, and 
also on their position, either among themselves, or with res- 
pect to the wires in communication with which they receive the 
electricity. The liquid mass interposed between the two poles 
of the pile is thus found to be in an electric state analogous to 
that of the atmosphere, that is, it presents a continuous medium, 
penetrated within with a free electricitj', which cannot escape 
from it on account of its imperfect conducting power, and which 
cannot spread itself there uniformly on account of its continued 
renewal from the source which developcs it. Except that 
in the atmosphere, when it is calm and pure, we always find the 
same kind of free electricity at all heights above the ground, 
which is analogous to the electric state of the moist band when 
the pile communicates with the ground by one of its poles ; while 
by insulating the pile, we obtain the two kinds of free electricity 
in different parts of the imperfectly conducting medium by which 
the two poles are united. The analogy by which 1 have passed 
from the use of a simple plate to that of a fluid mass, cannot be 
immediately verified by the application of the electroscope, at least 
it cannot be so without particular precautions. The great number 
of points of contact of the fluid w r ith the wires, and perhaps, also, 
the mechanical decompositions which take place, absorb so rapid- 
ly the electricity developed at each pole, and so much reduce its 
repulsive force, that it no longer gives any appreciable charge 
to the condenser. But this defect may be supplied by the indi- 
cations drawn from chemical phenomena. For instance, take 
four glass tubes, A, B, C, D, bent into the form of the, letter V ; 
pour into them a neutralized infusion of red cabbage; and 
after having placed them one after the other, connect them 
together by films of cotton or of amianthus, the ends being 
immersed in the liquid; then fix in the extreme tubes two 
wires communicating with the two poles of the voltaic pile. 
After some time, the liquor of the two siphons, A, J5, situat- 



1 60 Electricity, 

eel towards the vitreous pole, will become red ; that of the 
siphons C, Z), situated towards the resinous pole, will become 
green ; and their change of colour will take place progressively 
from the extreme parts towards the centre. The difference 
in the effects thus produced in the different parts of the same 
fluid medium by the electric current which passes through it, 
seem to prove that the two electricities are not uniformly dis- 
tributed. 

141. The electricity disposing of itself in this manner, we 
are furnished with a clue to a phenomenon, w T hich has not yet, 
I believe, been explained, although it has been justly regarded 
as one of the most remarkable hitherto presented by the appa- 
ratus of Volta. It consists in this, that if we suspend in the con- 
ducting medium, a wire placed more or less exactly in the di- 
rection of the transfer, this wire immediately manifests at its ex- 
tremities respectively a contrary electricity to that of the nearest 
pole, and all the chemical phenomena take place at these ex- 
tremities, which belong to the poles of the apparatus. For in- 
stance, recurring to the experiment just described, in which we 
employed four siphons filled with a neutralized solution of red 
cabbage, instead of uniting the liquid of the intermediate branches 
by means of films of cotton or amianthus, connect them by wires, 
and we shall see the liquid become red about the extremity of 
each wire which is directed toward the resinous pole, and green 
about the extremity of the same wire which is directed towards 
the vitreous pole. Now this is very easily explained according 
to the distribution of the electricity which we have attributed to 
Fig. 63 the fluid mass. For, supposing in the first place, for the sake of 
simplicity, that we introduce only a single wire AB; this wire is 
found to be subjected to electric influences of different degrees 
of intensity, the one, J 7 , exerted by the portion of the fluid mass 
which is situated towards the vitreous pole of the pile, the other, 
i?, by the portion situated towards the resinous pole. Now by 
inspection of the figure, it will be seen that these two actions 
o-onspire to decompose the natural electricities of the wire, and to 
decompose them in the same direction, so as to give it a resinous 
pole at the end opposite to the vitreous pole, and a vitreous pole 
at the end opposite to the resinous pole of the apparatus. It is 
precisely in this way, as we have seen, that a long wire insulated 



Chemical Effects of the Voltaic apparatus. 1 6 1 

vertically in the atmosphere, is affected, becoming resin- 
ous at its upper part, and vitreous at its lower, by the influ- 
ence of the unequal distribution of the electricity in the aerial 
mass. It is in this way, also, as we shall see hereafter, that a 
wire of soft iron placed near a magnetic bar, in a direction par- 
allel to the axis of this bar, becomes itself magnetic under the 
influence thus exerted. Now, since each wire placed in the 
voltaic current acquires, at its two ends, electric states absolutely 
similar, as to the kinds of electricity and its permanence, to 
what exists in the two wires which communicate immediately 
with the poles of the pile itself, it is very plain that the same 
power of decomposition must manifest itself here, and that each 
extremity of the interposed w r ire must produce the kind of ef- 
fect belonging to the particular electricity which it possesses. 
The same influence would be exerted equally upon any number 
of wires, suspended in this way one after the other in the volta- 
ic current, with different degrees of intensity, however, accord- 
ing to their position in the electrified mass. 

142. This in no degree explains why chemical compounds 
are decomposed under the influence of the electric current, 
nor why certain substances, when they become free, tend to- 
wards the vitreous pole, while others tend towards the resinous 
pole. These very remarkable facts depend undoubtedly on the 
general conditions which determine chemical combinations, with 
which conditions we are at present entirely unacquainted, since 
we are not able to say precisely in what this state of combina- 
tion consists. But the decomposition of a product being once 
supposed to have taken place, the unequal distribution of the 
two electricities in the conducting medium, as it seems to pre- 
sent itself to us, is sufficient to make the phenomenon of transfer 
clearly intelligible. Indeed, setting out from this result of ob- 
servation, that the transfer really takes place, let us consider, for 
instance, a particle of an acid which is travelling towards the 
vitreous pole. Since it tends thither and is carri d through the 
fluid which surrounds it, its march must be determined and reg- 
ulated by the electric charge reeehed at the instant, when it 
was separated from the combination in which it was before held. 
Now to produce this effect, it is sufficient that the charge in 
question should be resinous and stronger than that of the unde- 
E. fr M. 21 



162 Electricity. 

composed liquid particles which surround the wire fixed to the 
resinous pole ; for then the particle of acid will be repelled by 
the jjortion of the liquid mass near the resinous pole, and attract- 
ed by the portion situated toward the vitreous pole. Therefore, 
if it does not lose immediately its excess of electricity by com- 
munication, which, on account of the extreme weakness of 
this excess, it may not, since the liquid state of the medium, of 
which it also partakes, renders its motions there perfectly free, it 
must yield to the force of transfer which acts upon it, and follow- 
ing always the direction which the resultant of these forces im- 
presses upon it, must finally reach the vitreous wire, where it 
will deposit the resinous electricity which it possesses and re- 
ceive the degree of vitreous electricity belonging to the particles 
situated in this part of the fluid mass. It is then that we com- 
monly perceive new r chemical effects to take place, the opposite 
to those observed at the resinous pole. Gaseous products are 
disengaged, and new combinations formed ; but these effects, as 
inexplicable as the first, it does not belong to us to explain. 

Nevertheless I ought not to pass unnoticed some results of 
this sort which appear to indicate the conditions on which the 
possibility of chemical combinations generally depends. It has 
been found that an acid cannot be transferred through an alkali 
with which it forms an insoluble salt. May not this be an ex- 
tension of that influence which the contact of a solid body gen- 
erally exerts in the formation or the decomposition of certain 
products ? We know that water boils at a higher temperature 
in a glass vessel than in a metallic one ; thus the contact of the 
glass retards the disengagement of vapour, but the contact of 
particles of metal instantly determines this disengagement. 
When a rough piece of metal is thrown into a solution saturated 
with gas, it is generally by the points of the asperities that the 
disengagement of the gas takes place in the greatest abundance. 
When water is made to combine with several hundred times its 
volume of oxygen, as in the experiments of M. Thenard, the 
simple contact of a metal is sufficient to destroy this union with 
violence. May it not be in virtue of this general property that 
the wires attached to the two poles of the pile determine the 
combination and the disengagement of the substances with 
which they are in contact ? And may not this property depend 



Chemical Effects of the Voltaic Apparatus. 163 

en the power which thej possess of rapidly removing or commu- 
nicating electricity? May not the cessation of the transfer, 
when an insoluble combination takes place, during the passage 
of one of the substances, depend also on the same cause, namely, 
the forcible disengagement of the electricity with which the 
substances are charged ? And finally, may not the influence of 
this electricity to prevent the combinations which do not set it 
free, consist in this, that its repulsion for the electricity of the 
same kind with which the medium, traversed by it, is charged, 
joined to the force which repels it from one pole and attracts it 
to the other, prevents it from having a sufficiently intimate con- 
tact with the particles of the medium to enter into combination 
with them ? The examination of these different questions by 
experiment would form an interesting subject of research. 

By comparing the effects which take place in the same sub- 
stance under the influence of the voltaic current in the numerous 
combinations to which it may be subjected, we generally ob- 
serve a tendency to tranfer itself towards one or the other pole 
of the pile, and consequently to become charged with one or the 
other electricity. So true is this that in a great number of cases 
we can foretell that the decomposition of the product will be 
easy or difficult, and in what direction it will take place, that is, 
towards which pole each of the elements witi tend after separa- 
tion. Hence chemists have been led to suppose, with some ap- 
pearance of probability, that the decompositions which are pro- 
duced by the voltaic current, depend on this very tendency, 
which, being different in the elements of combination, urges some 
towards the vitreous pole of the pile, others towards the resinous 
pole, and causes them to separate in order that they may take 
these different directions when it is strong enough to overcome 
the affinity which unites them. It may be added, that the op- 
position of the electric state perhaps effects the separation other- 
wise than by the mechanical tendency which results from it 
towards each pole, as by destroying, for instance, some relation 
which must exist between the electric charges of a product, in or- 
der that the substances which compose it may remain combined ; 
for, being ignorant of the part which electricity may perform in 
the constitution of bodies, we ought to abstain from limiting hy- 
pothetically its effects. According to this way of considering the 



164 Electricity. 

power of decomposition exerted by the voltaic current, the pos- 
sibility of the phenomenon will depend in general on three cir- 
cumstances. (1.) The greater or less disposition of the princi- 
ples of the compound to take, in each particle, opposite electri- 
cal states ; (2.) The greater or less energy of these states ; 
(3.) The ratio of this energy to the chemical affinity which the 
principles of the substance have for each other. For instance, 
if we operate upon a body, the principles of which are easily 
brought into opposite electrical states of great intensity, then 
the pile may decompose this body, although the chemical 
affinity which unites its principles be very powerful. If, on the 
contrary, the affinity is xery weak, while at the same time the 
constituent principles of the substance have very little tendency 
to take opposite electrical states, it is very possible that decompo- 
sition will not take place. Finally, as in the friction of bodies 
against each other, there are some which take, at one time, vit- 
reous, and at another, resinous electricity, according to the 
mature of the rubber applied to them, so it may happen that the 
same chemical principle will at one time take the vitreous, and 
at another, the resinous state, according to the combinations into 
which it may enter ; and although generally each principle must 
possess in all combinations the same natural tendency, never- 
theless the final resliit will depend on the similar or different 
dispositions of the principles with which it is united. In all the 
experiments which h^e been thus far made with the voltaic ap- 
paratus, oxygen has apy eared to preserve that disposition toward 
the resinous state which it manifests in the decomposition of 
water, and which is also remarked in the experiments made 
with common electricity, where the oxygen of the air always 
tends to the surfaces electrified vitreously. Even when bodies 
are found to be composed of several principles, some of which 
have strong affinities for oxygen, this has communicated to them 
its resinous disposition, and drawn them towards the vitreous 
pole ; while the other principles have taken the vitreous state, 
and have gone to the resinous pole. In virtue of this law, all 
the oxides and the acids which contain oxygen, have been de- 
composed by the voltaic current, and the principle which is 
united with the oxygen, is transferred to the resinous pole ; and 
the oxygen, accoiding to its constant disposition, goes to the vit- 



Chemical Effects of the Voltaic Apparatus, 165 

reous pole. These interesting facts were first made known, as 1 
Lave already said, by MM. Hisinger and Berzelius. Sir 
Humphrey Davy, in varying and extending them, was led to 
try the action of the voltaic apparatus upon the alkalis, which 
had till that time been regarded as simple substances. He per- 
ceived what the philosophers of Europe have since witnessed 
with surprise and admiration, that bubbles of oxygen were dis- 
engaged at the vitreous pole ; while there appeared at the res- 
inous pole a number of brilliant globules of a metallic aspect and 
yet very light, which burned briskly in the air, and even pos- 
sessed the singular property of becoming inflamed in water. 
These were, therefore, the metallic bases of soda and potash, 
which were afterwards called sodium and potassium. But from 
the very nature of their properties, only minute portions of these 
substances conld be obtained, which were destroyed in the air 
as fast as they were formed. It was therefore necessary to 
seek for some means of preserving the^m from the contact of the 
air which consumed them. Dr Seebeck invented a very simple 
process for this purpose, which consists in combining sodium and 
potassium w r ith mercury as fast as it is disengaged. We make a 
hole in a small fragment of the hydrate of soda or of potash which 
is then filled with mercury ; we place this fragment on a metal- 
lic plate, and immerse in the mercury the resinous wire of a vol- 
taic apparatus, consisting of at least two hundred pairs of plates. 
We make the other wire communicate with the metal support; 
and the soda or potash is decomposed, as well as the water 
which it contains. The oxygen of both one and the other go to 
the vitreous pole, whither their electric state draws them. The 
hydrogen and the sodium or potassium thus abandoned, go to 
the resinous pole, where the hydrogen is disengaged in the form 
of a gas, and the potassium or sodium combines with the mercury 
which thus preserves it from the action of the air. From time 
to time we pour the amalgam into the oil of naphtha, and renew 
the mercury. When we have collected a certain quantity of 
amalgam, we distil it in a retort, with the least possible quanti- 
ty of air. The oil is evaporated first, and then the mercury, and 
at length the sodium or potassium remains free. In order that 
the decomposition of the potash may take place by the process 
which we have now described, it is necessary that these alkalis 



166 Electricity, 

should contain a sufficient quantity of water to transmit the elec- 
tricity of the pile, yet not so great a quantity that its decompo- 
sition shall require the action of all the electricity transmitted, 
for then the potash and the soda will not be decomposed. Sir 
Humphrey Davy and Dr Seebeck, by similar processes, 
were able to discover in the other alkalis the clearest evidence 
of decomposition. But it does not belong to a treatise like the 
present to enter into minute details upon such a subject ; I shall 
only add, that since the first discovery of Sir H. Davy respect- 
ing the composition of potash and soda, MM. Gay-Lussac and 
Thenard have succeeded in depriving these substances of their 
oxygen, by the simple action of chemical affinities. 

143. We have thus far considered the action of the pile only 
as decomposing bodies ; but it produces other very remarkable 
effects. For instance, if we make the communication between the 
two poles by very fine wires, and gradually bring them towards 
each other till they come in contact, an attraction takes place 
between them, which holds them together in spite of their elas- 
ticity ; the wires being of iron, a visible spark takes place 
between them, which, as we shall presently see, produces a real 
combustion of the iron. This phenomenon succeeds more cer- 
tainly, when we arm the extremity of one of the wires with a 
strip of gold leaf. This leaf is consumed at the point where the 
spark is seen. We may inflame detonating gases with this 
spark, and even phosphorus and sulphur, as with the spark 
drawn from our common electrical machines. 

We shall here speak only of the effects produced by the 
most common piles, of which the discs are a little larger than 
a dollar. But these effects, it is evident, will become much 
greater if we employ the same number of plates of a larger sur- 
face. For in piles in which the number of the elements and the 
nature of the moist conductors are the same, the thickness of 
the free electric stratum, upon plates of the same rank, is also 
the same, as we learn both from theory and experiment ; whence 
it follows that the whole quantity of electricity which these 
piles possess in a state of equilibrium, is exactly proportional 
to the surfaces of the plates ; and the same proportion also exists 
in a state of action, at least if we suppose that the conducting 
power of the interposed liquid is the same, and that this liquid, 



Chemical Effects of the Voltaic Apparatus. 167 

as well as the surfaces of the plates, undergoes, in the course of 
the experiment, only similar alterations. Thus MM. Gay-Lussac 
and Thenard found that the quantity of gas disengaged in a 
given time, is proportional to the surface of the plates, or, which 
is the same thing, to the whole quantity of electricity. The 
same proportion is observed in all other chemical effects. A 
pile with large plates, although composed of a small number of 
pairs, will ignite a certain length of iron wire. This phenome- 
non was observed for the first time by MM. Hachette and The- 
nard. The English philosophers, by giving to the voltaic appa- 
ratus a better form, which I shall describe hereafter, and uniting 
with the size of the plates, the increase of force which results from 
their number, have carried this effect to the highest degree. With 
their improvements, long wires of iron, of platina and other met- 
als are heated not only to redness, but until they are fused and 
resolved into globules ; and if they are made to pass during part 
of their course through liquids, these liquids may be heated to 
boiling. If instead of wires, we employ leaves beaten or rolled 
thin, they inflame and burn with different colours according to 
their nature. The sparks which are excited between the 
leaves or conducting wires, when they are brought nearly into 
contact, are so powerful as to become visible even in water. 
But nothing is more remarkable than the phenomenon exhibited 
when the conducting wires are terminated by points of perfectly 
dry charcoal. The great apparatus of the Royal Institution of 
London, which is composed of two thousand pairs of plates, four 
inches square, being prepared in this way, the spark began to 
dart from one charcoal pdnt to the other, when they were at 
the distance of about y 1 ^ of an inch. But soon after, the two 
points being brought to a state of high ignition, they might be 
removed from each other to the distance of four inches, with- 
out interrupting the light. The constancy of the electric 
discharges, between the two points, formed a continued jet of 
light bent in the form of an arc, of a splendor superior to any 
other flame, attended with so intense a heat, that the most re- 
fractory substances were fused, and globules of diamond and of 
plumbago disappeared as if they had suddenly evaporated. 
These effects were produced in the same way, and with still 
more energy, when the charcoal points were placed in air rarefi- 



168 Electricity. 

ed by the air pump. In this case, the stream of light continued 
to flow from one point to the other when the distance was no 
less than six inches ; and it might be continued whole hours with- 
out the charcoal being sensibly diminished. Hence it may be 
inferred with much probability, that the electric light is produc- 
ed in this case, as it is in ordinary electric explosions, by the 
passage of the electricity through the air, or the rarified vapours 
which separate the two points. The first discharge from one 
point to the other, must pierce this stratum of air or vapour, and 
for this reason it takes place only at a small distance ; but when 
it has once effected a passage, and divided, by its repulsive force, 
the particles of the surrounding medium, the following discharges, 
which meet with little interruption, tending all in the same direc- 
tion, easily make their way through the rarefied air, or through 
perhaps an almost perfect vacuum, which they have only to 
maintain, and they therefore take place at a greater distance. 
144. This continued production of light, and the analogous 
disengagement of light and heat which is observed in the wires 
when they are traversed by the voltaic current, are very re- 
markable phenomena ; and the more so because, in the case of 
the wires, when they are placed in a vacuum or in gases with 
which they cannot enter into combination, the ignition may be 
supported for whole hours, and be renewed as often as we choose, 
without any diminution of their weight. It is extremely dif- 
ficult, not to say impossible, to conjecture whence is derived 
the light and heat thus produced. Will it be said that the 
light is disengaged by the compression which the electric current 
causes the substances upon which it acts to undergo ? But then, 
since the current is continuous, it would seem that the compres- 
sion, being once exerted, ought to continue during the whole 
time of the experiment ; and thus we could at most attribute to 
it the first appearance of the light, but not its continued produc- 
tion. Can it be, that the two electric principles, in com- 
bining with each other, -produce light immediately? We are 
not acquainted with any phenomenon which would oblige us to 
regard this supposition as impossible, or even as improbable. 
The following experiment tends to confirm it. When a voltaic 
trough is charged with a saline solution or with an acid diluted 
with water, we observe that its chemical action is immediately 



Chemical Effects of the Voltaic Apparatus. 169 

is at its highest point of energy, and that in a few instants, it 
rapidly decreases, so as to become, after some hours, very feeble, 
or nearly insensible. We shall soon see the cause of this de- 
crease ; it is here stated simply as a fact. Now if, at the moment 
of the most powerful action of the apparatus, we interpose be- 
tween its poles the longest iron wire of a given diameter which 
it is capable of heating to redness, we shall soon find that it is 
no longer suificient to heat a wire of the same length, and that 
it goes on diminishing until an iron wire, however short, will 
discharge the apparatus completely without any appearance 
of ignition. Let us now suppose, that, instead of shortening 
successively in this way the interposed wire, we keep it always 
of the same length, we shall find that the continually de- 
creasing portion which suffers ignition, is situated at the middle 
of the wire ; so that at the moment of the last possible igni- 
tion, it will be found to take place precisely at the middle of the 
conducting wire, where in fact it appears that the union of the 
two electric principles must be most abundant. 

145. In all which precedes, we have supposed the apparatus to 
eonsist of a considerable number of plates ; but the ignition may be 
produced with a single pair by rendering the thickness and the 
lengtf of the wire very small compared with the extent of sur- 
facf oelonging to this pair of plates. For instance, we take a 
rectangular plate of zinc ZZ, about two inches in breadth, 
by six in length. We wrap about it a plate of copper CC, from Fi S- 
which we separate it by rolls of resin, in such a way that the zinc 
shall not touch the copper at any point. The zinc plate has on 
one of its sides, an appendage, m, of the same metal, to which is 
fixed a copper rod *, directed parallel to the length of the plate ; 
another copper rod t', fixed to the exterior surface of the cop- 
per plate, rises in a direction perpendicular to the rod t, so that 
the extremities of the two rods are about a fourth of an inch dis- 
tant from each other. We join these extremities by a platina wire 
/, of the same length and of about one four-thousandth of an inch 
in diameter. This system evidently forms a voltaic pair, one 
of the elements of which is the zinc plate, and the other is the 
system of the copper rod Z, of the platina wire/, of the rod t', 
and finally, the large copper plate CC. The developement of 
electricity is produced by the contact of the rod t, with the 

E. fr M. 22 



1 70 Electricity. 

appendage m. Now suppose the whole apparatus suspended 
by a non-condiicting rod attached to this appendage ; the zinc and 
the copper have no communication with each other, except by 
the surface in contact at?n t ; consequently there will be no circu- 
lation of the electricity. But this circulation will be possible if 
we interpose between the large plates C, Z, some moist conductor, 
as a saline solution, or what is still better, a mixture of one part 
by bulk of nitric acid, one of sulphuric acid, and fifty or sixty 
of water. Indeed, when we immerse a portion of the surface 
of the large plates in such a mixture, we perceive a lively effer- 
vescence to take place immediately in the conducting liquid ; 
and in a few moments, the platina wire interposed between 
the rods / 1\ is heated to redness. This state of ignition con- 
tinues for a long time, especially if we give the conducting li- 
quid free access to the zinc plate, by making openings O, O, O", 
in the lower part of the copper plate ; and when it has ceased, 
it may be made to re-appear, by substituting fresh liquid for 
that which has been used. Besides, it will be readily perceived, 
that the dimensions here attributed to the combined plates, are 
not absolute, but merely relative to the diameter and length of 
the wire to be ignited. By greatly diminishing its length and 
diameter, the wire might be heated to redness with a pair of 
plates of much smaller size. Dr Wollaston has carried this to 
the extreme by employing as a conductor an exceedingly fine 
platina wire, when very small copper and zinc plates are suffi- 
cient to form the apparatus ; and upon being immersed in an 
acid mixture, the wire, which was at first almost invisible on ac- 
count of its fineness, becomes manifest by its ignition. 

This experiment presents in its details, some particulars 
which, at first view, it may appear hard to reconcile with the 
idea, that the developement of electricity which takes place, re- 
sults from the simple contact of the metals. As the explanation 
of these apparent anomalies depends upon modifications produced 
in the passage of the electric current, by the more or less perfect 
conducting pow r er, it will naturally find a place toward the close 
of the following section. 



Action of the Voltaic Apparatus upon itself. 1 7 1 



Examination of the Changes which take Place in the Voltaic Ap- 
paratus by its Action upon itself- — effects which hence result in 
its Electrical State. 



146. The chemical action of the voltaic apparatus is not 
exerted at the extremities merely of the wires, by which the 
communication is established between its two poles; it occurs 
also between its metallic elements, the moist conductor which 
separates them taking the place of the liquid in which the wires 
are immersed. Hence result, in the very interior of the appa- 
ratus, considerable changes which affect its electrical state, 
either by changing the conditions of equilibrium in the con- 
tact of the elements of the pile, or by altering the conducting 
power. 

The first effect of this action, is a rapid absorption of the 
oxygen of the air which surrounds the apparatus. We may as- 
certain this in a very simple way, by placing a vertical pile 
upon a support surrounded with water, and covering it with a 
receiver, the base of which descends into the water. In a few 
moments, the water is seen to rise in the interior of the receiver, 
especially if we establish a communication between the two 
poles of the pile by wires, so as to cause the circulation of the 
electricity through it. When there is no communication estab- 
lished, an absorption still takes place, but much more gradually. 
In all cases, after a certain time, depending on the size of the 
pile, and the quantity of the surrounding air, the absorption 
ceases, and the air which remains under the receiver no longer 
presents any traces of oxygen. This phenomenon was discov- 
ered by M. Frederick Cuvier and myself, soon after the voltaic 
apparatus became known in France. It was attended with a cir- 
cumstance worthy of note ; namely, that as long as there remain- 
ed any oxygen to be absorbed, the chemical and physiological 
effects of the apparatus still continued, although with decreasing 
intensity ; so that if the conducting wires attached to the two 
poles be made to return from under the receiver, in tubes of 
glass, they may be used to decompose water and communicate 
shocks to the organs. But all these effects cease, when the 



172 Electricity. 

surrounding oxygen is exhausted. By a natural conse- 
quence, the chemical and physiological action of the same pile 
is much more lively and more durable when it is surrounded 
with pure oxygen, than when it is enclosed with an equal bulk 
of common air; and even in the latter case, when by the pro- 
gress of the absorption, the pile is found immersed in an atmo- 
sphere of nitrogen, and has become entirely extinct, the intro- 
duction of a small quantity of oxygen is sufficient to restore it. 

147. When we disconnect the pile which has thus been kept 
in action for several days, under a receiver filled with atmos- 
pheric air or oxygen, with a communication constantly estab- 
lished between the poles, we find that the metallic discs which 
compose it adhere to one another and to the intermediate pieces 
of cloth with such force, that it requires some effort to separate 
them. When detached, we perceive that the chemical action of 
the pile, has reacted upon itself, and has produced remarkable 
changes in its, own elements. If the pile were composed in 
this way, zinc, moisture, copper, zinc, &c, and placed upon its 
zinc base, we constantly find that particles of each piece of zinc 
have been detached, and transferred to the copper of the pair next 
above; and if the copper and zinc elements of each pair are simply 
placed the one upon the other, so that they may be separated, we 
also find that particles of the copper of each pair have gone to the 
piece of zinc next above. If this arrangement of the pile is invert- 
ed, the order being copper, moisture, zinc, copper, &c, the copper 
descends upon the zinc beneath, and the zinc upon the copper, 
from the top to the bottom of the column. The direction of the 
transfer is inverted with respect to a perpendicular ; but it re- 
mains the same as to the order of the elements of which the ap- 
paratus is composed. 

According to this arrangement, it is necessary that the zinc 
in order to reach the copper should traverse the piece of moist 
cloth which separates them. In piles, where the communica- 
tion has not been established, this transmission does not take 
place, the surface of the copper is smooth, and that of the zinc 
which is opposed to it is only covered with small black threads, 
which follow the direction of the threads of the cloth. When the 
communication has been established a short time, particles of 
oxide begin to pass, and go to the copper. Finally, if the action 



Fi 



Action of the Voltaic Apparatus upon itself. 173 

is strong, the surface of the copper becomes entirely covered. 
Then the chemical and physiological action of the pile ceases, 
either because the oxide of zinc, deposited upon one of the faces 
of the copper, and the metallic zinc which touches the other face, 
exert the same electrical influence in the contact ; or because 
the interposition of this layer of oxide presents too great an ob- 
stacle to the transmission of the electricity, or more probably 
from these two effects combined. 

Sometimes the oxide of zinc, after having traversed the piece 
of cloth, returns to the metallic state upon the copper. Then 
the parts of the piece of copper upon which this precipitation 
takes place are in contact with zinc at both surfaces. The 
inequality of the electric state at those surfaces ceases, therefore, 
with respect to these parts, and they no longer act in the pile 
except as neutral conductors ; and this prevents the parts of the 
same piece of copper, which the zinc, thus transferred, has not en- 
tirely covered, from preserving with the piece of zinc which 
touches them at the' other face, the general relations of elec- 
tric equilibrium which take place in contact, and from thus de- 
veloping the same quantities of electricity as before. 

148. The motion of transfer being from the zinc to the cop- 
per through the moist conductors, when the copper tends to the 
zinc, it is always where the faces touch each other immediately. 
Then if the copper adheres to the fcinc, and preserves its metallic 
brilliancy, brass is sometimes formed. These precipitations 
take place only when the communication is established between 
the extremities of the pile. It is also necessary, in order that 
they may occur, that the cloth discs should not be too thick, nor 
of too close a texture. 

These, I believe, were the first phenomena of transfer which 
were observed with the voltaic apparatus. M. F. Cuvier and 
myself, announced them in the work of which I have spoken 
above ; but we have not seen their general application. Their 
theory is evidently the same as that of the other chemical de- 
compositions which take place between the poles of the pile. 
Nevertheless, there is this difference, that the wires attached to 
these poles carry into the substances in which they are immers- 
ed, electricities of different kinds, the one vitreous, the other res- 
inous ; while the metallic pieces which follow each other imme- 



174 Electricity. 

diately in the voltaic apparatus, have electric charges of the 
same nature, and merely unequal in intensity. Such an inequal- 
ity, being perpetually renewed as it is in the interior of the pile, 
is therefore sufficient to establish between the simple principles 
which separate these pieces, and in the matter of the pieces 
themselves, a tendency to separation similar to that produced by 
the electricities of different kinds ;■ and in fact the influence of 
these unequal charges upon the substances interposed must pro- 
duce a separation of their natural electricities, which reduces 
things to precisely the same state in the two cases. Il is wor- 
thy of remark that these phenomena of interior transfer are 
particularly sensible in piles composed of plates of a very small 
diameter. The reaction of these piles upon themselves is beyond 
comparison greater and more rapid than that of piles with large 
discs. 

149. All these interior changes being well determined, it is 
necessary to examine the influence which they may have upon 
the electric state, and afterward upon the permanence of the 
chemical action, of the voltaic apparatus. 

Let us begin with the absorption of oxygen, by means of 
which the chemical and physiological energy of the pile is in- 
creased. It is evident that this increase would not take place, 
if the conducting power were perfect ; for then each metallic 
element of the pile would draw from the ground, instantly and 
directly, the quantity of electricity necessary to it according to 
the place it occupies. Thus the pile would continually recharge 
itself to the same degree, as soon as it was discharged ; and thus 
would necessarily maintain the constancy and the continuity of 
its action. But the experiments related in the preceding sec- 
tion have taught us that this case of perfect conducting power, is 
in fact, imaginary; and although it may be useful to suppose it, 
in order to understand distinctly the increase of electricity by 
the superposition of the metallic pairs, it is necessary to qualify 
these suppositions, on account of the imperfection of the conduct- 
ing power, in order that we may fully understand the pile as it 
is actually formed. 

150. According to Volta, oxygen can act only by establish- 
ing a more intimate communication between the metallic ele- 
ments of the pile, linking them, as it were, to each other, and to 



Action of the Voltaic Apparatus upon itself. 1 75 

the imperfectly conducting cloths which separate them by oxida- 
tion. It is indeed probable, that this adherence contributes to 
augment the conducting power, especially at the commencement 
of the action. But when it has become so strong that the whole 
pile forms, as it were, only a solid mass; when the moist cloths, 
interposed between the discs, have become dry ; when all the 
oxygen which surrounds the pile has been absorbed, and the 
chemical action seems entirely extinct, what new degree of ad- 
herence can the introduction of a new quantity of oxygen pro- 
duce ? And especially, how could such an effect take place instan- 
taneously ? This last circumstance evidently excludes all idea of 
a simple mechanical cause ; and proves that the restoration of 
the electricity depends on the mere presence of oxygen between 
the metallic pairs ; either because oxygen immediately restores 
to each of the pairs, by its mere contact, the electric charge 
which its place requires, or because it suddenly re-establishes the 
conducting power, by the combination which it forms with the 
substances composing the pile. 

In order to determine precisely the conditions on which this 
restoration must depend, let us imagine a pile composed in this 
way, copper, zinc, moisture, and let us make it communicate 
with the ground by its copper base. In a state of equilibrium 
the several pieces of this pile will have an excess of vitreous elec* 
tricity, depending on the place which they occupy. If we touch 
the upper piece, the excess which it possesses will flow off into 
the ground, and it will tend to resume this excess from the lower 
pieces through the moist conductors. But these conductors not 
being perfect, a certain time will be necessary for this effect ; if 
we repeat the discharge before the communication can have 
taken place, the upper piece will receive vitreous electricity 
from the piece of copper which it immediately touches, so that 
the latter will acquire an excess of resinous electricity ; the same 
thing will happen more or less to all the metallic pairs. Such 
must, therefore, be the state of the pile in which the moist con- 
ductors interposed between the pairs have been so modified by 
the effect of a free communication established for a long time 
between the two poles, that the transmission of the electricity will 
no longer take place, or it will take place too slowly to produce 
the chemical and physiological phenomena. 



1715 Electricity. 

This being laid down, let us now introduce about the discs an 
atmosphere of oxygen. This oxyg-en will be attracted by all 
the pieces of zinc which are in the vitreous state ; it will, there- 
fore, combine with their substance in virtue of the affinity exist- 
ing between them, and of the electric influence by which it is 
determined. But the oxide of zinc hence resulting will, in its 
turn, be attracted towards the surface of the piece of copper next 
above, which the imperfection of the conductors leaves in the 
resinous state. It will therefore carry to this piece the vitreous 
electricity of the metallic zinc which it abandons ; and this mo- 
tion of transfer continued from the top to the bottom of the pile 
will re-establish the transmission of the electricity. The same 
thing will also happen in a pile communicating with the ground 
by its zinc end, because the imperfect state of the conductors 
would in the same way permit the metallic elements to take 
opposite states. 

This explanation, which is due to Sir H. Davy s applies 
equally to all the other chemical decompositions which take 
place in the interior of the pile. The products which result, 
being attracted towards surfaces differently electrified, trans- 
fer with them the electricity of these surfaces, and produce 
directly the same result that would arise from a perfect conduct- 
ing power. 

151. Nevertheless, granting that this motion of transfer must 
contribute to the re-establishment of the electric equilibrium, it 
is difficult to admit that it is the only cause ; for it appears that 
it can only act gradually and slowly, especially in an apparatus, 
where, by the effect of a long communication between the poles, 
the moist cloths have become completely dry. It is not im- 
possible, therefore, that the oxygen also contributes to the 
re-establishment of the equilibrium b\ its contact merely, in vir- 
tue of a decomposition produced in its natural electricities by 
the contact of surfaces electrified vitreously. This presents an 
important subject of inquiry. Moreover, whatever may be the 
mode in which the pile is thus quickened, it must be subject to 
this essential condition, that the two electricities are developed 
or transmitted at the same time, and in equal quantities, that 
is, in quantities capable of mutually neutralizing each other. 
For in endeavouring to collect, by the condenser, the excess of 



Action of the Voltaic Apparatus upon itself, 177 

the one or the other of the electricities which may be developed 
by the electric action in the most powerful piles, where the com- 
munication between the two poles was established by wires, 
I have ascertained that it was insensible* on the application of 
the most delicate tests. 

152. In recapitulating the facts which have now been stated, 
we see that all the modifications which take place in the chemi- 
cal state oi the moist conductors must affect the action of the 
pile, either by altering the conditions of the electric equilibrium 
in the contact, or by modifying the conducting power. On account 
of these two causes, the quantity of electricity communicated to 
the condenser by a single contact may undergo considerable va- 
riations. This is, in fact, directly confirmed by experiment, and 
is still more apparent from the great inequalities of chemical ac- 
tion which the same piles present at different moments, a cir- 
cumstance to which we shall soon have occasion to recur. 

The progressive and inevitable loss of power of the elec- 
tromotive apparatus mounted with moist conductors, has led 
electricians to make a great number of attempts to discover a 
construction of the pile requiring only perfectly dry conductors. 
Thus far their efforts have been fruitless, or at least, piles thus 
constructed, have not possessed a conducting power sufficient to 
produce chemical decompositions, which is the principal object 
for which a permanent apparatus is wanted. 

With respect to this question, Volta discovered among metal- 
lic substances a very remarkable relation, which, if it be as exact 
as he supposes, renders the construction of the pile with these 
substances impossible. I shall explain it in his own way ; I 
have not had an opportunity to verify it myself. 

If we arrange the metals in the following order; silver, cop- 
per, iron, tin, lead, zinc, each of them will become vitreous 
by contact with that which precedes, and resinous with that 
which follows. The vitreous electricity will, therefore, pass 
from the silver to the copper, from the copper to the iron, Irom 
the iron to the tin, and so on. 

Now the property in question consists in this, that the in? 
equality of the electric charge between the silver and the zinc is 
equal to the sum of the differences which belong to the metals, 
comprehended between them in the series. Hence it follows, 

E. b M. 23 



178 Electricity. 

that, placing them in contact, in this order, or in any other that 
we choose, the extreme metals will always be in the same state 
as if they touched immediately. Consequently, if we suppose 
any number of elements to be thus disposed, of which the ex- 
tremities are silver and zinc, for instance, we should have the 
same result as if the elements were merely formed of these two 
metals, that is, there will be no effect, or it will be the same as 
that which a single element would have produced. 

153. It has appeared thus far, that the preceding property 
extends to all solid bodies which are very good conductors ; but 
ii does not subsist between these bodies and liquids. It is for 
this reason that, we succeed in the construction of the pile, by 
means of liquids. Hence results the division which Volta made 
of conductors into two classes, the first comprehending solid bo- 
dies, and the second, liquids. We have as yet been able to con- 
struct the voltaic apparatus only by a proper mixture of these 
two classes ; with the first only it is impracticable, and we are 
not sufficiently acquainted with the mutual action of the bodies 
which compose the second, to say, whether it be rhe same with 
them or not. It would seem, however, that it is not, for nature 
has constructed true liquid piles in the electric apparatus of 
certain fishes, particularly of the torpedo. This apparatus, 
which is situated near the stomach of the animal, is composed of 
a multitude of tubes or cells arranged by the side of each other 
and filled with a peculiar liquid. It appears that the animal can 
put this pile in action at will ; and it then communicates true elec- 
tric shocks to the living bodies, with which it is in contact. . It 
even appears, if what is related be true, that it possesses the 
power of sending its charge to a distance through water. 

154. If we have not succeeded in forming absolutely dry and 
undecomposible voltaic piles, we have been able to obtain those, 
the action of which, although very feeble, is at least of long con- 
tinuance.. Such is the pile which M. Hachette constructed with 
metallic pairs of plates separated by a simple stratum of flower 
paste mixed with sea salt. When this stratum is dry, the 
moisture which it draws from the atmosphere renders it suffi- 
ciently conducting to permit the re-establishment of the electric 
equilibrium between its metallic elements, in a space of time 
.? ensibly instantaneous ; it also charges the condenser by a sim- 



Action of the Voltaic Apparatus upon itself. 179 

pie contact in a sensible instant, and preserves this property for 
whole months, and even years, which makes it a true electro- 
phorus ; but it gives no shock, does not affect the taste, or pro- 
duce any chemical action. M. Zamboni has also constructed a 
pile of which the electric effect appears very durable ; he formed 
it of discs of paper gilt or silvered on one side, and covered on 
the other with a stratum of pulverized oxide of manganese. In 
the arrangement of the discs the metallic pairs are formed of 
gold or silver in contact with the oxide of manganese. The inter- 
posed paper serves as a conductor. Hence results a very feeble 
transmission of electricity ; and we obtain merely electrical 
signs, as with the paste pile, but no chemical action or physiolo- 
gical effect. This latter class of phenomena requires, therefore, 
a more rapid re-establishment of the electric equilibrium. To 
prove the great effect of a retardation in this particular, I have 
constructed piles, in which the place of the moist body was sup- 
plied by discs of nitrate of potash melted in the fire ; in this 
case the conducting power was so feeble that the condenser re- 
quired a sensible time to become charged, and continued to 
increase its charge to a certain limit, which charge was the same 
as with the most powerful piles for a similar number of pairs. From 
the law of these charges I have concluded that the initial quan- 
tity of electricity, given by such a pile to the condenser, in an in- 
finitely small space of time, was incomparably less than that given 
by the ordinary pile ; and as it is these initial charges which 
produce the chemical decompositions, when the communication is 
established between the two poles, we see why piles in which the 
conducting power is very feeble do not produce these phenomena, 
and are not attended with any chemical action, taste, or shock. 
155. This same consideration explains also, why voltaic 
piles which at first exert a powerful chemical action, when the 
metallic pairs which compose them have just been placed in con- 
tact with conducting liquids, lose their power very fast and soon 
produce only very feeble effects, although the condenser, by 
touching their poles, always takes quantities of electricity sensi- 
bly equal. It is because this contact, however rapid it may be, 
is never absolutely instantaneous ; and unless the conducting 
power is very much weakened, as in the pile with discs of nitrate 
of potash, it continues long enough in each case to permit the 
condenser to acquire the maximum charge which it is capable of 



180 Electricity. 

receiving. But the gradual progress of this charge, although 
insensible to us, is not therefore the less real, and may have been 
incomparably more rapid at the commencement of the action ; 
so that the initial discharges of the apparatus might then pro- 
duce phenomena which they are afterward not in a state to pro- 
duce. 

156. Making use of these observations, we perceive that the 
most favourable arrangement of the voltaic apparatus for pro- 
ducing powerful chemical effects, is that in which the electric 
charges developed instantaneously and continually by the con- 
tact of the metallic plates of each pair, shall pass in the freest man- 
ner possible, through the' liquid conductors which separate them. 
It will, therefore, be necessary, in the first place, to choose those 
liquids which transmit the electricity most perfectly ; such ap- 
pear to be the nitric and sulphuric acids diluted with a large 
quantity of water. It will also be useful to employ metallic 
plates of a large surface. This large extent is not necessary 
indeed, in the parts where the two metals of each pair mutually 
touch each other; for it appears that the electricity developes 
itself there instantaneously, and spreads itself with so much 
rapidity, that the smallest surface in metallic contact is sufficient 
to maintain the most extended liquid masses and those of the 
greatest conducting power under a given repulsive force. 
But for this very reason, in the contact of the plates with the 
liquid, the extent of surface will have a very great influence 
upon the absolute quantity of electricity transmit! ed in equal 
limes, and therefore, the effects will increase with the dimen- 
sions. Finally, the liquids interposed should be kept, a» far as 
possible, in their primitive state of composition, or of conducting 
power, while the apparatus is in use; and as this condition can- 
not be fulfilled by merely increasing the quantity, which would 
render the apparatus inconvenient in practice, and would even be 
injurious, if we augmented the intervals by which the metallic 
pairs are separated, it is necessary to provide means by which the 
liquid conductors may be easily renewed, and brought in con- 
tact with the plates only at the moment when we wish to make 
use of them. We obtain all the advantages above mentioned by 
forming the apparatus of a series of double plates similar to that 
i45 already described. We fix all the pairs parallel to each other 



Action of the Voltaic Apparatus upon itself. 181 

upon the same piece of wood, made strong enough to support 
them without bending ; and we arrange below an equal number 
of wooden, porcelain, or glass troughs, filled with the conducting 
liquid. If we wish to make use of the apparatus, we lower the Fig. 68. 
wooden bar, and each pair is immersed in the corresponding 
trough. When our experiments are completed, we raise the 
bar, and the troughs filled with liquid remain prepared for an- 
other experiment ; but if we think that the liquid needs to be 
renewed, we empty the troughs and fill them again. Experience 
has proved, that of all the arrangements at present in use, this is 
the most simple, convenient, and efficient.! 

157. Voltaic piles have been constructed of wires of a single 
metal, bent in the form of an arc and immersed in vessels filled 
with a single liquid, under which are placed lamps alternately, 
by which they are heated. It is said that the mere inequality 
of temperature thus established between the two branches of 
the same wire is sufficient to cause an unequal electric charge. 
I have not had an opportunity of verifying this fact ; but it ac- 
cords fully with the general theory of Volta, if we adopt the 
explanation which has been given of his experiments. 

158. I shall conclude this section with an account of some 
remarkable phenomena, which are very easily explained upon- 
the principles we have established relative to the influence which 
the conducting power has on the effects of the voltaic pile. We 

take two wires, A, Z, one of silver, the other of zinc, and im- Fi S- 69 * 
merse them both in a very weak solution of sulphuric or muriatic 
acid. As long as the two wires do not touch in any point, the zinc 
dissolves in the acid and disengages hydrogen, while no gas bub- 
bles appear upon the silver wire. But if we bring the two wires 
into contact, at their dry extremities, then gas escapes from each 
of them. This is a very simple phenomenon. Until the contact of 
the two wires takes place, no derangement is produced in the 
equilibrium of their natural electricities ; but the contact being 
established, the derangement takes place, and an electric current 
passes from one to the other through the conducting liquid. 
Thus far there is nothing irreconcilable with the other phenom- 
ena. But what follows seems more extraordinary. If we bend 



$ See note on Hare's Deflagrator and Calorimotor. 



182 Electricity. 

the silver wire so that its immersed extremity shall also touch 
the zinc, or even if it be soldered to it so as to form a continued 
ring, half silver, and half zinc, the same effects still take place. 
Nevertheless the zinc wire is then in contact with the silver by its 
two ends; and according to the theory ofVolta,the electromotive 
actions, exerted at these two ends should counteract each other, 
and hence it ought to remain in its natural state, which is con- 
trary to observation. But this contradiction disappears, if we 
give, as before, to the fundamental experiments, their true inter- 
pretation, independently of any hypothesis. We have seen that 
these experiments indicate simply a state of electric equilibrium, 
which must take place in the contact of the metals with each 
other, in virtue of which, the silver in contact with the zinc 
ought to have, lor instance, an excess — e of resinous electricity, 
and the zinc an excess -j- e of vitreous electricity. But this 
condition must be satisfied in the ring at the two points of junc- 
tion of the silver wire with the zinc wire ; therefore, the same 
electric state extends to each wire, on account of the free circu- 
lation of the electricity through their substance. Now, if we 
immerse the ring in a conducting liquid below the points of 
junction of the two wires, so that a portion of each wire shall be 
immersed, the two opposite electricities which these portions 
possess will unite through the conducting liquid ; and as they 
are incessantly renewed at the points of contact of the two 
metals, there results a continued circulation which ought to pro- 
duce all the phenomena of a voltaic pair. This case is abso- 
#ig- 71- lutely the same as that of a complete plate, of which the upper 
half is of zinc, the lower of silver, and which is immersed in a 
conducting liquid below the point of junction. In such a plate, 
however, the inequality of the electric charge obtains for all the 
points of the line of contact AB ; and it is hence communicated 
by the conducting power to the whole of each plate ; whereas, 
in the ring, this inequality originally exists only in two points, 
which are the points of junction of the wires. 

159. Soon after the voltaic troughs were constructed the two 
opposite sides of each trough were formed of the metallic plates 
themselves, which gave the arrangement represented in fig. 72, 
in which the letter Z indicates the zinc plates, C the copper, 
and L the liquid interposed. Now it often happens, that after 



Action of the Voltaic Apparatus upon itself. 188 

having poured the liquid into the troughs, the disengagement of 
the gases causes it to overflow their upper edges, which are thus 
moistened so that each zinc plate communicates with the con- 
tiguous copper by a moist stratum. Nevertheless, the effects of 
a general current are still produced, although with less energy 
than when the edges of the plates are kept dry. This is because 
the communication thus established by each moist stratum is far 
from being sufficient to transmit all the electricity developed by 
the contact of the entire surfaces of the plates. The remainder 
passes, therefore, through the liquid of the troughs, and, being 
incessantly renewed by their contact, causes in the usual way a 
continued electrical current. 

Hence, it is evident, that this current would still exist if each 
piece of zinc were brought in contact with each piece of copper 
by a better conductor than a simple moist stratum ; except that 
its effects would thus be still more weakened, the quantity of 
electricity being less. For example, take a single pair of plates Fig. 58. 
having a large surface like those already described, and immerse 
it in an acid mixture ; it will cause a red heat in the platina wire 145. 
which communicates from the copper to the zinc. This being 
done, interpose somewhere between the two plates of zinc and cop- 
per, another small wire, as/', bent in such a way, as to sustain 
itself between the two plates by its elasticity. It will then be 
seen, that notwithstanding this communication the platina wire 
/ is still red hot, although in a less degree ; or if we choose, we 
can cause another wire of less diameter to become entirely red. 
This is because the communication established by the second 
wire is not sufficient to transmit all the electricity developed by 
the contract of the whole surface of the plates ; and the remain- 
der, passing through the first wire is still sufficient to cause its 
ignition ; in the same way as in traversing the liquid conductor, 
this remainder produces a disengagement of the gas ; if any 
doubt existed of the division which thus takes place in the elec- 
tric current between the two wires/,/ 7 , we may assure ourselves 
of it by this circumstance, that the wire/, the simple pressure 
of which causes a less perfect communication, becomes itself 
sensibly warm, while the wire/ is red hot. These curious ex- 
periments were communicated to me by M. Gay-Lussac, to 
show that the theory of Volta required to be modified, and I 



184 Electricity, 

have been led to refer them to simple conditions of electric equi- 
131. librium as heretofore explained. Upon the same principles it 
may he shown also, why the action of a pile, connected by a 
liquid of great conducting power, does not cease to act when 
it is immersed entirely in water. It is because the electricity 
circulates through the water less rapidly than in the interior of 
the pile; whence it follows, that the communication established 
by the water cannot entirely discharge it, so long as the inter- 
posed liquid remains. 



Of Secondary Piles. 

160. While all sorts of combinations were tried for the pur- 
pose of forming voltaic piles entirely of dry, and consequently 
unalterable substances, Ritter discovered one, which although 
incapable of developing electricity by its own action, is never- 
theless susceptible of being charged by the voltaic pile in such 
a way as to acquire, for a moment, all its properties. This is 
called the secondary pile of Ritter. 

To form a just idea of this pile, it is necessary to call to 
139, mind an observation of Volta, already mentioned, and which 
proves the imperfect conducting power of vegetable substances 
saturated with water. If we insulate an electrical column, of 
which the upper pole is vitreous and the lower pole resinous, 
and if we make these two poles communicate by an imperfect 
conductor, as a strip of paper, for instance, moistened with 
pure water, each half of this strip will take the electricity 
of the pole with which it communicates. The upper part will be 
vitreous, and the lower, resinous. We have remarked that this 
phenomenon is an evident consequence of the laws by which 
the electric principle is governed, when distributed over bodies 
which transmit it imperfectly. 

Let us now suppose that we remove this imperfect conduct- 
or, with a nonconducting body, as a glass rod ; the equilibrium 
will not be instantaneously established between its two extremi- 
ties ; they will remain for some time, the one vitreous, the other 
resinous, as when they communicated with the two poles of the 
pile. 



Secondary Piles. 1 85 

These differences will gradually diminish, according as the 
opposite electricities re-combine, and their neutralized actions- 
will soon become altogether insensible. 

It is to this, precisely, that the fundamental experiment of 
Ritter refers itself; except that he substitutes for the moist strip 
of paper a column composed of copper discs and moistened 
pasteboard alternately. This column is incapable by itself of 
putting the electricity in motion, at least if we suppose its ele- 
ments of each kind to be homogeneous in themselves ; but it 
becomes charged by communication with the pile, like the band 
of moist paper of which we have spoken. Nevertheless there 
is an essential difference in the two results. It appears that the 
electricity, when it is feeble, meets with some difficulty in passing 
from one surface to the other. This seems at least to result 
from the experiments of Ritter, and perhaps the resistance is 
produced by the imperceptible stratum of nonconducting air, 
which adheres to the surfaces of all bodies. The electricity 
introduced into the column, composed of a single metal, meets 
therefore with a similar difficulty in passing from the metal to the 
moist pasteboard ; this obstacle increases according as the alter- 
nations are more numerous. Thus a pile once charged must 
lose its electricity very gradually when there is no direct com- 
munication between its two poles. But if we establish this com- 
munication by a good conductor, the passage of the two electric- 
ities, and their combination, taking place rapidly, will cause a 
discharge, as in the Leyden jar. A new state of equilibrium 
will follow this effect, in which the repulsive forces of the diffe- 
rent plates will be diminished in the ratio of the quantity of 
electricity which is instantly neutralized. The discharges 
must therefore be repeated with diminished effects, according to 
the number of contacts ; but they soon cease to be sensible 
on account of the equal charge which they tend to establish be- 
tween all parts of the apparatus. In a word, the action of the 
column depends on this, that it becomes a better or worse 
conductor, according as its two extremities do or do not commu- 
nicate with each other. 

As to the manner in which the electricity arranges itself in 
this case, it must be such, that its repulsive force at the surface 
of each plate, combining with the resistance, of the contiguous 
E. <&■ M. 24 



186 Electricity. 

surfaces, shall produce an equilibrium with the united actions 
of all the others. Consequently, if we suppose the number of 
elements unequal, and the whole apparatus insulated, the quan- 
tities of electricity will go on decreasing from the two extremi- 
ties where they will be equal and of contrary signs, as in the 
primitive pile, to the centre where they will be nothing ; but if 
the apparatus communicates by its base with the ground, the 
electricity will go on increasing throughout the column, from the 
base where it will be nothing, to the summit where it will be 
equal to that of the primitive pile. 

161. The apparatus which we have described is less power- 
ful than the ordinary pile in producing shocks, the decom- 
position of water, and the other physiological and chemical 
effects. By varying the number and order of the paste- 
board and copper discs, Ritter obtained several interesting 
results. Thus he observed, that of all the ways in which we 
can dispose of a certain number of heterogeneous conductors, 
the arrangement in which there are the fewest alternations, is 
the most favourable to the transmission of electricity. For in- 
stance, if we construct a pile with sixty four discs of copper and 
sixty four of moist pasteboard, arranged in three groups, in such 
ft way that all the pasteboard discs shall make a continued series 
terminating each way with thirty-two metallic plates, this pile will 
conduct very well the electricity of Volta's column, and conse- 
quently will receive very little, if any, permanent charge. If we 
interrupt the moist conductors by a copper plate, the conducting 
power is immediately diminished. Prlore frequent interruptions 
weaken it still more ; and by thus multiplying the interruptions, 
we arrive at systems in which the conducting power is hardly 
sensible. It was from these phenomena that Ritter learned the 
resistance which a feeble electricity suffers, in passing from one 
surface to another ; a resistance which has no effect except in 
this state of weakness ; for, by a singular property, an electricity 
strong enough to overcome it opens for itself a free passage, and 
flows off entirely. 

162. We have seen that by changing the distribution of the 
elements in a secondary pile, we can change at will its conduct- 
ing power. It was natural to suppose that these modifications 
would diversify the chemical and physiological effects. To de- 



Secondary Piles. 187 

termine their progressive operation, Ritter varied the arrange- 
ment of a given number of moist and solid conductors from their 
separation into two groups, to the greatest number of alterna- 
tions. The following are the results which he obtained. 

A very small number of alternations is easily traversed by 
the electric current of the primitive pile, supposing it to be of 
sufficient strength. The apparatus does not, therefore, become 
charged permanently; and the chemical and physiological 
effects disappear. By multiplying the alternations, the prim- 
itive pile remaining the same, the secondary pile begins to be 
charged. It communicates- electricity to the electroscope; 
it disengages bubbles of gas from water, but it produces no 
shocks. The number of alternations being still further increased, 
the electric charge increases, and we obtain the decomposition 
of water, the effect upon the taste, and the shocks. But beyond 
a certain number of alternations, the chemical and physiological 
effects no longer increase, although the whole electric charge 
should remain the same, or even continue to augment ; this limit 
being passed, the charge sustains itself; but the other effects 
diminish ; the disengagement of the bubbles ceases first, and 
afterwards the shocks. We then find our apparatus at the 
other extreme of a too imperfect conducting power; and the 
progression according to which these phenomena disappear, the 
electric charge remaining constant, completely proves what we 
have said above of the manner in which they depend on the 
velocity of transmission. 

163. We see from the same principles why the apparatus of 
Ritter is more proper than any other for exhibiting separately 
these two sorts of action. In the common pile, the quantity of free 
electricity increases with the number of pairs, and balances the 
resistance which results from the alternations ; whereas in the 
secondary pile, the repulsive force of the electricity at the two 
poles can never exceed that of the primitive pile ; the resistance 
arising from the alternations is entirely employed in modifying 
the escape of the same quantity of electricity. 

Finally, if the column of Volta is capable of thus charging the 
secondary pile of Ritter, it owes this property to the circumstance, 
that the repulsive force of the electricity at its poles is extreme- 
ly feeble, and, as it were, imperceptible. A stronger electricity. 



3 88 Electricity. 

such for instance, as that of the common electrical machine, would 
entirely traverse the system of conducting bodies which form 
the secondary pile, and would consequently be incapable of pro- 
ducing any of the effects which result from its accumulation. 

164. The difference which is found to exist in the chem- 
ical action of the common pile, on account of the size of the 
plates, takes place also in the secondary pile. The nature of the 
pasteboard discs, their thickness, the nature of the solution with 
which they are moistened, finally, the order in which they are 
arranged, and a variety of other little circumstances, modify 
these effects in a thousand ways, which it would be both curious 
and useful to examine. 

The secondary pile being formed, as we have said above, 
with a single metal and a moist substance, it would seem, at first 
view, that it ought not by itself to have any electricity; and in 
fact, its own proper action, before it has been charged, is scarce- 
ly appreciable. But we can, nevertheless, render it sensible, 
by placing the muscles and nerves of a frog in contact with its 
two extremities. This depends probably on a slight dissimilar- 
ity which must inevitably exist between a considerable number 
of plates although formed of the same metal. 



On the unequal Resistance which the two Electricities, when -eery 
weak, meet with in traversing different Bodies, 



165. In examining the manner in which electricity discharg- 
es itself through bodies of different kinds, we have observed that 
those bodies which conduct best, oppose, nevertheless, a sensible 
resistance to its passage. . Comparing these results with those 
presented by imperfectly insulating supports, we were led to con- 
clude that the imperfection of the conducting power would become 
more and more sensible, according as we diminish the repulsive 
force of the electricity transmitted, so that at a certain degree of 
weakness, all bodies, even the metals themselves, would cause 
a perfect insulation. The voltaic apparatus, furnishing an inex- 



The two Electricities subject to unequal Resistance. 189 

haustible source of electricity, with a very small repulsive force, 
is well suited to experiments of this kind ; and it also shows us 
differences and imperfections in the conducting properties of li- 
quids, which our common electrical machines fail to point out. 
M. Ermann applying himself to inquiries of this sort, made 
the curious discovery that there are certain substances, the con- 
ducting power of which is not the same for the vitreous as for 
the resinous electricity ; so that diminishing more and more the 
repulsive force, we find a limit, where the body becomes a non- 
conductor for one, while it is a conductor for the other. 

M. Ermann insulated a voltaic apparatus, put up with a good 
liquid conductor, as a solution of the muriate of soda, for instance, 
and caused e tch of its poles to communicate with a very sen- 
sible gold leaf electroscope, also insulated. Each electroscope 
soon acquired the degree of divergence belonging to the num- 
ber of plates, and the electric zero was at the middle of the ap- 
paratus. 

This* being done, he took a prism of very dry alkaline soap, 
and inserted into one of its ends a wire communicating with the 
ground. If now the other end of the prism be made to touch one 
of the poles of the pile, this pole is immediately discharged ; the 
divergence of the electroscope, connected with it, ceases, while 
the electroscope at the other pole diverges still more. The 
whole takes place just as if the pole, touched by the prism, had 
-communicated with the ground, and the soap seems to perform 
the office of a conductor for either electricity indifferently. 

Now the pile remaining always insulated, and the repulsive 
forces of its poles being re-established, he caused these poles to 
communicate with each other, by means of the same soap, in- 
serting into the two ends of the prism, wires proceeding from 
the two poles. Notwithstanding this communication, the two 
electroscopes will continue to diverge as before, so that the soap 
seems in this case to perform the office of a non-conductor. 

But when this insulation is well ascertained, touch the soap 
for an instant with a wire which communicates with the 
ground ; the resinous pole will be immediately neutralized, and 
the repulsive force of the vitreous pole will attain its maximum. 
Thus the soap resumes its conducting power, but only to suffer 



1 $0 Electricity. 

the resinous electricity to flow oft'; and it is always that elec- 
tricity which it prefers to transmit, even when we make the 
contact near the . wire which communicates with the vitreous 
pole of the pile. This pole does not, on this account, remain 
the less insulated. 

The flame of alcohol presented to M. Ermann similar effects; 
but the conducting power was jn favour of the vitreous electricity. 
All this is to be understood only of very feeble degrees of electric- 
ity, such as those furnished by the electromotive apparatus ; for 
soap and the flame of alcohol would conduct stronger degrees of 
electricity, imperfectly it is true, but in a manner sensibly 
equal. 

In repeating these experiments, sulphuric ether is found to 
exhibit a property which completes the discovery of M. Er- 
mann. This liquid being interposed between the two poles of 
the pile seems to insulate them like the soap and alcohol. If 
we place within the circle of communication, an apparatus for 
the decomposition of water, no bubbles are disengaged ; indeed 
we have all the signs by which the two poles are known to be insu- 
lated. But if for a single instant, we touch the ether with a wire, 
to form a cummunication with the ground, applying at the same 
time a condenser to either of the poles of the pile, this conden- 
ser is completely charged, as if the ether had suddenly become 
a conductor of the kind of electricity which belongs to the pole 
to which the condenser is applied. 

166. In relating these experiments, I have said that the two 
poles of the pile appeared to be insulated by the interposition of 
a prism of alkaline soap. The insulation is indeed only par- 
tial. The prism of soap does not absolutely prevent all motion 
in the electricity ; it only renders it much slower than in the pile 
itself, which permits the pile to become sensibly re-charged, and 
to acquire a tension at its poles while the soap is discharging 
it. A proof of this is, that the same prism of soap conducts all 
the electricity of a pile having a less conducting power, -such as 
the paste pile, for it takes absolutely all tension from its poles, so 
that the condenser no longer becomes charged by touching them. 
The flame of alcohol interposed between the poles of this same 
pile does not so completely discharge it. It suffers a tension to 



The two Electricities subject to unequal Resistance* 191 

remain, and with it we can repeat the experiments of M. Er- 
mann. This flame does not conduct the electricity so well as 
the alkaline soap. These experiments are given in detail in the 
Bulletin des Sciences, for 1816, p. 103. 



MAGNETISM. 



General Phenomena of Magnetic Attraction and Repulsion, 

167. Most of the fragments of iron ore in which a degree of 
oxidation has taken place, are found to possess, when taken from 
the earth, the singular property of attracting iron, by an invisible 
power. This attraction is often so feeble that it is necessary to 
employ the most delicate processes in order to render it sensible ; 
but it is sometimes strong enough to support considerable weights. 
The mineral is then called a magnet] from the Greek word [tayvris ; 
and hence the term magnetism is used to stand for the phenom- 
ena of attraction exhibited by this mineral. 

168. The most simple method of showing the power and 
distribution of magnetism in a piece of natural loadstone, is to 
roll it in iron filings, and afterwards to withdraw it from them. 
It will then be seen that different quantities of these filings will be 
attached to different parts of its surface. This effect is particularly 
sensible in two opposite points, JV, 5, where the filings are accu-Fig. 76. 
mulated in the greatest abundance, standing as it were on end 
nearly parallel to each other. These parts are called the poles of 

the magnet. In order to observe their properties more easily, 
we shall suppose that the loadstone is cut by two plane and par- Fig. 77. 
allel faces, A, jB, in a direction nearly perpendicular to that of 
the small filings. The following phenomena will then be observed. 

Each of the poles, presented to the iron filings, will attract • 
them at a distance, in the same manner that a stick of sealing 
wax, when rubbed, attracts all bodies that are presented to it. 
If we suspend horizontally a small needle of iron or steel, by an 

t The name loadstone is also applied to it, from the Saxon word 
Icedan, to guide. 

E. & M. 25 



194 Magnetism. 

untwisted linen thread or fibre of silk, or any other sufficiently 
flexible substance that will allow it to move with full liberty, 
each pole of the loadstone will attract it, and cause it to oscillate 
about its centre. This power is exerted with equal force through 
both conductors and non-conductors of electricity. Its action is 
not intercepted by water, glass, paper, or flame ; insulation is un- 
necessary, and the loadstone loses nothing of its virtue by being 
touched. 

Of the nature of the principle which produces these phe. 
nomena, we are entirely ignorant ; but to avoid circumlocution, 
we shall designate it by the name of magnetism, in the same way 
that we give the name of electricity to the unknown principle of 
electrical phenomena, and the name of caloric to the equally 
unknown principle of heat. It is necessary, in order to proceed 
philosophically, to attribute to this unknown principle only the 
properties and qualities which are indicated, or rather which are 
rendered necessary by the phenomena which it produces. 

169. If we place the polar surface A, of one loadstone, suc- 
cessively in contact with the surfaces A' and B', of another, we 
shall find that it attracts one of them, B', for example, and repels 
A' \ and reciprocally, the polar surface B of the first loadstone 
attracts A' and repels B '. The mutual tendency of the attracting 
faces shows itself, not only by their adherence when they touch 
each other, but also by the effort exerted when they approach 
near each other. The repulsion is not so easily recognised in 
this manner ; but we may render it sensible, by placing one of 
the loadstones on a piece of cork floating upon water ; for, as it 
is then at liberty to move, if we present to it the other loadstone 
it will approach to it or recede from it, according as it is attract- 
ed or repelled. 

We see, therefore, from this experiment, that the powers ex- 
erted by the two polar surfaces of a loadstone are not similar, 
since the one attracts what the other repels, and vice versa. The 
most simple way of expressing this result, will be to distinguish 
magnetism into two kinds, differing, if not in their physical 
essence, at least in the external and apparent mode of their 
action. It is thus that electrical attraction and repulsion lead 
us to distinguish electricity into two kinds, namely, the vitreous 
and the resinous, which have received these names from the 



Magnetic Attraction and Repulsion. 195 

substances in which they are developed ; and it is of importance 
to remark, that the two magnetisms reside in the opposite poles 
of a loadstone, in the same manner as the two electricities reside 
in the opposite poles of a heated tourmaline. 

1 70. If we examine the crest of filings attached to the poles of 
a loadstone, we shall observe that their radii are composed of sev- 
eral parcels of filings, adhering end to end to one another. This 
phenomenon is particularly deserving of attention, as it teaches us 
that iron placed in contact with a loadstone, becomes itself mag- 
netic, in the same manner that an insulated body becomes elec- 
trical when placed near another body that is electrified. 

In order to establish this property, we take several bars of 
soft or malleable iron, such as is used for keys. After we are 
satisfied that none of these bars possesses any perceptible mag- 
netism, which may be determined by their not attracting iron 
filings, we suspend one of them a b to one of the poles B Fig. 78. 
of a loadstone ; the lower end b of this bar will immediately 
acquire all the magnetic properties. If we now place it among 
iron filings, they will adhere to it, and we may even suspend to 
it a second bar a'6', and to this a third bar a"6", as represented 
in the figure. All these will adhere to one another, till their 
total weight exceeds that which the loadstone is capable of sup- 
porting. As soon as the first bar a b detaches itself thejr will 
all separate and fall; and if we again try to unite them, they 
will be found no longer capable of supporting each other. They 
preserve, however, in general, some feeble remains of magnetism 
which will become sensible by placing them in filings of iron, or 
presenting them to iron needles freely suspended. This tran- 
sient communication of magnetism will still take place, even if 
the first bar, without touching the loadstone, is kept at a dis- 
tance from it by the interposition of a piece of card, or a plate 
of glass ; but the total weight thus supported at a distance is 
much less, and the magnetic attraction decreases very fast as 
the distance increases. 

171. If instead of soft iron, we employ bars of steel, or iron 
hardened by the hammer, the adherence of these bars to one an- 
other is less easily and less readily effected, but it is more du- 
rable; and the bars when separated from the loadstone, pre- 
serve the magnetism which they have acquired from being in 



196 Magnetism. 

contact either with one another or with the magnet. The 
soft iron and the steel employed in these experiments have the 
same relation to each other as a rod of metal, and a stick of 
sealing-wax, when submitted to the influence of an electric body. 
In the metal the decomposition of the natural electricities is sud- 
den, but the recomposition is equally so, and it takes place as 
soon as the metal is withdrawn from the influence of the electri- 
fied body. In the wax, on the contrary, the natural electricities 
are separated with difficulty but when the separation is effected, 
they experience the same difficulty in their re-union, and the elec- 
tric state continues after the action of the electrified body has 
ceased. 

Magnetism may be communicated to a bar of steel in a more 
prompt and energetic manner by two loadstones than by one, 
the two extremities being placed in contact at the same time 
with opposite poles. The same loadstone may thus successively 
render magnetic any number of bars, without losing any portion 
of its original virtue, from which it follows that it communicates 
nothing to the bars, but only developes by its influence some 
hidden principle. In the same manner a stick of sealing wax, 
when rubbed, loses nothing of its electricity by the decomposi- 
tion which its influence effects at a distance in the natural elec- 
tricities of other bodies. 

1 72. If, after having magnetized in this way a steel bar or wire, 
we suspend it horizontally by an untwisted thread or bundle of 
silk fibres, or make it float on water by placing it on a small 
piece of wood or cork, it will not turn indifferently to every point 
of space, but it will take a determinate direction, which in Eu- 
rope is nearly north northwest and south southeast. We say in 
Europe, for in certain parts of the earth, the north extremity of the 
bar deviates from the meridian towards the west ; in others towards 
the east ; while there are some in which it coincides with the 
meridian itself. This deviation is called the declination of the 
magnetic needle or the variation. It is constant at the same mo- 
ment in every place; and all magnetic wires thus suspended 
freely will take directions truly parallel ; but this common 
direction varies with the time, according to laws derived from 
observation. The vertical plane in which the magnetic needle 
directs itself at any given place is called the magnetic meridian. 



Magnetic Attraction and Repulsion, 197 

because it does not deviate much from the astronomical meridian, 
in those parts of the globe which were formerly most frequently 
visited ; but it is now found that in certain places, particularly 
in the polar regions, the declination of the needle becomes very 
considerable, and reaches even to 90° ; so that the needle 
directs itself towards the true east and west, instead of turning 
to the north and south. 

173. When several magnetic needles are thus freely sus- 
pended in a horizontal position, such of their extremities as turn 
to the same terrestrial pole are those which, in the magnetising 
process, have been in contact with the same pole of the magnet, 
and which have consequently received a magnetism of the same 
k'nd. If these extremities are made to approach, they will 
mutually repel each other ; while, on the contrary, if the ex- 
tremities which have received different kinds of magnetism are 
made to approach, they will mutually attract each other. In 
this respect, the two kinds of magnetism have the same effects 
as the two kinds of electricity. 

When we hold one of the poles of a loadstone at a distance 
from a magnetic needle, suspended horizontally by its centre, 
the two poles of the loadstone act at once upon the needle ; but 
the action of the nearest pole is always the strongest. The 
needle then turns towards the loadstone the pole which is at- 
tracted, and keeps at a distance the one which is repelled. If 
after it has taken a position of equilibrium, we turn it ever so 
little from its position, it will return to it by a series of oscilla- 
tions, in the same manner as a pendulum drawn from a vertical 
line will return by the influence of gravity. A motion abso- 
lutely similar to this is observed in magnetic needles freely sus- 
pended, when they are drawn ever so little out of the magnetic 
meridian. From this circumstance, therefore, as well as from the 
constant direction which they take, we infer that they are acted 
upon by the terrestrial globe as by a true magnet ; whether this 
faculty is owing to the number of mines of iron and magnetic sub- 
stances contained in the earth, or whether it depends upon some 
other cause still more general. Hence we are furnished with con- 
venient names for the two kinds of magnetism, the one being called 
boreal, which resides in the northern part of the globe, and the 
other austral, which resides in the southern ; and therefore, in 



1 9S Magnetism. 

order to preserve the analogies of attraction and repulsion, 
we must consider the extremities of the bars or needles which 
point to the north, as south poles, and the extremities which 
point to the south, as north poles. 

174. The preceding experiments clearly indicate the direc- 
tion of the vertical plane in which the resultant of all the mag- 
netic forces is exerted at any particular place ; but it still re- 
mains for us to determine the absolute direction of this resultant 
in the plane itself. In order to this, take a cjdindrical needle of 

Fig. 79. steel a 6, provided with an axis passing perpendicularly through 
its middle point. When the needle is suspended by its centre 
upon well-polished planes, and accurately balanced so as to re- 
main in any position indifferently in which it is placed, let it be 
carefully magnetized. Then upon being placed upon its sup- 
ports in the magnetic meridian, it will no longer remain indif- 
ferent with respect to its position as before, but one of its poles, 
namely, that which possesses austral magnetism will incline itself 
to the horizon, at least in Europe ; and after a few oscillations it 
will settle at a determinate angle. This angle is called the mag- 
netic inclination, or the dip of the needle ; and it is different in 
different places. Near the terrestrial equator, there is a zone 
where the needle placed in the magnetic meridian is horizontal. 
To the south of this zone, the extremity which possesses the 
boreal magnetism inclines downwards ; to the north, that which 
possesses the austral magnetism ; and this indicates two kinds of 
forces, the one austral and the other boreal, which are predomi- 
nant on different sides of the equator. 

In order to measure accurately the magnetic inclination, the 
axis of suspension of the needle is placed on the centre of a ver- 

Fi*. 80. tide circle of copper MM whose limb, divided into degrees, moves 
upon a vertical axis VV, so that it may be brought into every 
possible azimuth. The axis VV itself is placed in the centre of 
a horizontal circle, divided in a similar manner, which serves to 
determine the direction in which we turn the first circle MM. 
This apparatus is called a dipping needle. We shall soon point out 
the precautions to be observed in magnetizing and suspending the 
needle, and also in measuring its inclination ; but this cannot be 
understood till the laws of magnetism are established. 



Magnetic Attraction and Repulsion. 199 

When the direction of the resultant of the magnetic forces 
exerted by the terrestrial globe is thus ascertained in any par- 
ticular place, its action may be instantaneously exhibited by a 
very striking experiment. Suspend a magnetic needle a 6 by its Fig. 8*. 
centre, with a number of untwisted fibres of silk, placing it in a 
small paper box and balancing it by a small weight on the 
south branch, so that it may have perfect liberty to move in a 
a horizontal plane. Now, since the needle will naturally be in 
the magnetic meridian, and will lie there in a state of rest, take 
a bar AB of soft unmagnetized iron about five feet long and four- 
tenths of an inch square, and, inclining it nearly in the direction 
of the magnetic inclination, hold its lower end A near the north- 
ern extremity of the needle, and a repulsion will immediately 
take place. If, on the contrary, the upper end B of the bar is 
held to the northern end of the needle, by making the bar descend 
parallel to itself, as in figure 82, an attraction will take place. 
Hence it is obvious, that in this inclined position, the bar of iron 
is suddenly magnetized by the magnetic influence of the globe, 
in the same manner as it would have been by the influence of 
any other loadstone that might be presented to it ; the lowest 
half of the bar nearest the earth acquiring a magnetism contrary 
to that which prevails in our hemisphere, namely, austral, 
and the upper half acquiring the opposite kind, namely, bo- 
real magnetism. The two ends A, £, of the bar are therefore 
in the same state as the two ends ct, 6, of the needle, w 7 hich were 
directed towards the same terrestrial poles, and it is from this 
cause that there is a repulsion when a and A are held near one 
another, and an attraction when a and B are brought together. 
In order to shew that these phenomena really depend on the 
sudden communication of magnetism to the bar, in consequence 
of the position in which it is held, we have only to reverse the 
two ends, while its inclination remains the same. In this case, 
the under and upper ends of the bar will exhibit the same 
phenomena that have been already described ; and therefore the 
phenomena will be opposite to those which the same end of the 
bar manifested before. The magnetic poles of the bar are 
then suddenly interchanged by being reversed ; and it is in order 
that this may be effected instantaneously, that we have employ- 
ed a bar of soft iron, and not a bar of steel or hard iron. 



200 Magnetism* 

To this same cause is to be attributed the magnetism which 
the iron crosses of spires, and other bars of this metal, acquire, 
by being kept a long tiaie in a vertical position The terrestrial 
globe magnetizes them also by its influence. The effect would 
be transient, if the iron which composes these bars were quite 
soft; but the hammering necessary to give them their shape, and 
even the action of the air, continued for a long time, communi- 
cates, particularly to the parts near the surface, a considerable 
degree of hardness. The magnetism in this case is not impress- 
ed instantaneously ; time is necessary for its developement by 
the action of the globe ; but, for the same reason, the magnetism 
is permanent when it is once produced. * According to Gilbert,t 
this remark was first made upon the bar of the weather-cock 
on the church of the Augustines, at Mantua. Others attribute 
the first observation of the fact in question to Gassendi, who no- 
ticed it on the cross of the church of Aix, in Provence ; but, 
with regard to the theory of the phenomenon, which is the most 
important point, it seems to belong solely to Gilbert. 

The directive property of the loadstone is one of the finest 
discoveries ever made by man ; it gives to navigators an infalli- 
ble method of recognising the direction of their track across the 
boundless ocean, in the darkness of night, and when fogs or tem- 
pests entirely obscure the heavens. A magnetic needle, balanc- 
ed upon a pivot, points out the course to be pursued ; and this 
valuable indication is as fully to be relied on, as even an obser- 
vation of the stars. Previously to this useful and simple dis- 
covery, which was not made till the twelfth century, the sailor 
could not venture to a distance from the coast. The compass 
has enabled him to launch into the ocean itself, and to seek 
new regions, unknown to the most powerful nations of antiquity. 

It is with this, as with most other useful inventions ; we are 
ignorant of the person to whom society owes such an invaluable 
gift. We do not even know precisely what nation was the first 
to employ the polarity of the needle as a means of obtaining a 
fixed direction in space. The Jesuit missionaries assure us, 
that they formerly found among the Chinese, traces of this 

|Dr William Gilbert, an English Physician, the friend of Bacon, 
who, about the year 1600, wrote a book upon magnetism that displays 
much talent. 



Magnetic Attraction and Repulsion. 201 

method, which belong to a very remote antiquity ; "but they 
supposed that it was employed merely as a guide on land ; and 
that the Chinese had never thought of using it at sea, a thing 
much more important, without doubt, but which might have been 
less so to a people whose navigation seems to have been always 
very limited. But, be this as it may, we find evident proofs of 
the existence and nautical use of the compass, in Europe, towards 
the year 1150. 

Such are the leading phenomena of magnetic attraction and 
repulsion ; but, before reducing them to a general theory, we 
must attend to some other details, which could not have been 
sooner introduced without interrupting the general train of our 
reasoning. 

175. It was long believed that iron and steel were the 
only substances that could be rendered magnetic ; but it has 
lately been found, that nickel and cobalt possess the same 
property. After these metals have been purified by very 
accurate chemical methods, needles may be formed of them ca- 
pable of being magnetized and of directing themselves in the 
magnetic meridian very energetically, though with less force 
than needles made of steel ; but from the nature of the process 
employed in the preparation of thesg metals, it is impossible to 
suppose that their action is due to the imperceptible portion of 
iron which may still remain in them. We shall soon see what 
is the origin of the magnetism attributed by several philosophers 
to copper, and some other bodies. 

176. A magnetic bar of whatever metal loses its virtue when 
it is brought to a white heat. Not only is it incapable, when in 
this state, of attracting iron, but even if the iron itself is a mag- 
net, it is not itself attracted ; it remains insensible to the action 
thus exerted. This fact, which was known to Gilbert, may be 
confirmed in a very simple manner, namely, by placing the piv- 
ot of a small compass needle in a good spirit lamp, with one 
or more wicks, and surrounding the whole with a cylindrical 
glass to prevent any agitation from the external air. After hav- 
ing placed the needle horizontally upon a pivot, so that it can 
be shewn to be sensible to the action of a loadstone, or a mag- 
netic bar, placed near it, let the lamp be lighted. The needle 
enveloped in the flame, will soon become red hot ; and if in this 

E. & M. 26 



202 Magnetism. 

state the magnetic bar is again presented to it, the needle 
will feel its influence, whether it is red, or bluish red ; but when 
it reaches a white heat, it will become completely insensible to 
the presence of the magnet. 

This result being obtained, remove the loadstone to a dis- 
tance ; and after having left the needle a short time exposed to 
the heat, extinguish the flame, and the needle will soon cool and 
become dark. But if, during this process, it is found to be 
pointing in a direction not exactly perpendicular to the magnetic 
meridian, it will have recovered some traces of magnetic power; 
and this power will be the more sensible, according as the nee- 
dle is less or more remote from the magnetic meridian. Hence 
we may conclude, that this power has been restored to it by 
the influence of the earth itself. We see, therefore, that in the 
progressive cooling of the needle, there is a particular tempera- 
ture at which it becomes sensible to the magnetic action, while 
it preserves sufficient ductility and softness to be affected even 
by a very feeble force ; after which, the increasing hardness, 
produced by farther cooling, renders it fit to preserve, in all im- 
aginable positions, the developement of magnetism which is thus 
produced. This experiment, so remarkable for its consequences, 
is found in Dr Gilbert's W|>rk.* Dr Hook employed the same 
means for impressing magnetism upon bars of steel, by placing 
them in the direction of the magnetic meridian, at the moment 
when, after being suddenly heated, they were tempered in cold 
water ; and Dr Robison rendered the operation still more per- 
fect, by substituting, in place of the weak action of the terrestrial 
globe, that of two powerful magnets, placed at the two ends of 
the red hot bars, at the instant they are plunged in water. Dr 
Robison informs us, that they thus acquire a considerable de- 
gree of magnetism, a result quite conformable to the theoretical 
notions that may be deduced from the process of magnetizing 
bars, which will be soon explained. On this account, it may be 
important to try the method anew, as it may be found useful in 
magnetizing bars of a large size. 

177. In this manner of operating, as well as in those which 
we have before described, the magnetism is developed either 

*Lib. iii. cap. 12. 



Magnetic Attraction and Repulsion. 203 

by the influence of a magnet, or by that of the earth ; but it 
appears that it may also be instantaneously excited by different 
mechanical means, as by the blow of a hammer, by pressure, 
by torsion, and by electrical discharges. 

Having taken, for example, a common iron wire of two or 
three lines in diameter, and from 10 to 15 inches long, bend it 
by resting one of its ends upon a plate of iron, or rather put it 
through an opening in a thick iron plate, and bend and twist it 
in different directions till it is broken. It will be found to have 
acquired the magnetic virtue, as may be seen from its attracting 
iron filings, or from its attracting one end of a needle and re- 
pelling the other, when the twisted extremity is presented to it. 

The same effect may be produced upon a rod of hard iron, 
by holding it in a vertical position, and striking its upper end 
slightly with a hammer. That the phenomenon may be very 
sensible, it is necessary that the rod be two or three feet long; 
and if it is afterward reversed, and the blows repeated upon its 
other end, it will gradually lose the magnetism impressed upon 
it, and will, by continuing the process, acquire a contrary mag- 
netism, its poles being reversed. The same effect may be pro- 
duced by letting it fall vertically upon a hard body. The uten- 
sils used by locksmiths almost always become magnetic, by the 
repeated blows to which they are subjected. Scissors, knives, 
and ulmost all cutting instruments, are more or less so, particu- 
larly if they have been employed in cutting iron. In order to 
show their magnetic influence, they should be presented to a 
small magnetic needle, suspended horizontally by a single fibre 
of the spiders web, or by an untwisted fibre of silk, enclosed 
within a glass receiver to prevent any agitation from the air. 
The smallest magnetic force will thus attract one extremity of 
the needle, and repel the other. By this means it is proved, 
that every piece of iron which has suffered any friction becomes 
megnetic, and that an electrical discharge, acting like a blow, 
developes magnetism in iron wires through which it is made to 
pass. Lightning produces a similar effect upon the mariner's 
needle, and sometimes even reverses its poles. Perhaps, indeed, 
these different methods produce their effect by agitating the 
particles of the metal and thus disposing it to receive the influ- 
ence of the terrestrial magnetism. 



204 Magnetism* 

From these facts we might be led to conjecture, that the 
communication of magnetism consists in a particular kind of dis- 
placement effected among the particles of a bar of iron or steel. 
In order to determine this, M. Gay-Lussac endeavoured to as- 
certain if these metals undergo any change of dimensions when 
they become magnetic. He took a hollow tube of iron AB shut 
up at the two ends, and to one of these ends he fitted a tube of 
glass extremely fine, and divided it into equal parts. He then 
introduced water into this apparatus till the tube of glass was 
partly filled ; and having waited a certain time till the tempera- 
ture of the liquid became uniform, he magnetized the iron tube. 
The surface of the water in the small tube did not experience 
any displacement, so that this change of state did not produce 
in the iron any appreciable change of bulk. 

It is equally established, by means of the most exact balan- 
ces, that the iron does not suffer any sensible alteration in its 
weight, in consequence of being magnetized; a result which 
might have been anticipated, from the striking analogy which 
subsists between magnetic attraction and repulsion and those 
which are produced by the equally imponderable principle of 
electricity. 

The degree of proximity which exists among the particles of 
iron, nickel, and cobalt, has a great influence upon the facility 
with which they are rendered magnetic. These metals, when 
they are pure and perfectly ductile, do not retain their magnet- 
ism, but acquire and lose it instantaneously. They may be 
made, however, to preserve it, either by mechanical means, 
such as pressing, twisting, or rolling them ; or, as has been ob- 
served by M. Gay-Lussac', r by combining them chemically with 
substances not magnetic, as carbon, phosphorus, arsenic, and tin. 
As the proportion of these substances increases, the magnetism 
is communicated with more difficulty, and it also lasts longer ; 
but at last there arrives a limit, when it is no longer possible to 
develope it in any sensible degree, and then the combination 
appears to be no longer capable of magnetic attraction. This 
property is only weakened, however, to a great degree without 
being entirely extinguished. For we can in this same state 
still obtain magnetic effects by means of more delicate tests 
that we soon shall make known. 



Magnetic Attraction and Repulsion. 205 

From these phenomena, we should be led to infer, that 
whatever alters the state of aggregation of the particles of the 
metals, exerts an influence upon their magnetic properties. The 
effect of temper is of a similar nature. To perceive the reason 
of this, we need only to be reminded of what it consists in. 

178. When a bar of steel has been heated to redness, and 
allowed to cool slowly in the air, its particles, in approaching 
nearer and nearer to one another, take the distances and the po- 
sitions of a stable equilibrium, to which they are gradually 
drawn by the slow and progressive effect of their reciprocal at- 
attractions. This is called the annealed state. But if we 
plunge the red hot bar into a fluid which cools its surface 
suddenly, the particles of this surface will take at first hur- 
ried arrangements, to which they are forced by this sudden 
change ; and having thus become immovable, they form a 
a kind of crust, to which the molecules of the interior of the mass 
are also constrained to adapt themselves with rapidity, in pro- 
portion as the cooling reaches* them. Hence there results a 
kind of crystallization different from a stable equilibrium, as 
may be observed in Prince Rupert's drops, which are nothing 
else but tempered glass. Pure metals are incapable of acquir- 
ing temper, and the cooling, whether slow or sudden, does not 
alter their physical properties. Thus soft iron remains soft 
after being suddenly cooled ; but iron, combined with carbon, 
and thus converted into steel, is changed by this operation, 
becoming more hard, more elastic, and more frangible, and 
to a greater degree, according to the suddenness with which 
it is cooled. Such a process may naturally be presumed 
to have an influence upon the magnetic properties, as it in fact 
has. The magnetic metals are magnetized with more difficulty 
when they are tempered, than when they are not ; but the mag- 
netism being once communicated, it is retained much longer. 
The difficulty of magnetizing increases with the hardness or 
temper ; and this hardness has an influence also upon the inten- 
sity of the magnetism which the substances in question are ca- 
pable of acquiring. 

As the temper depends upon the difference of temperature to 
which the metal is subjected, it is important to find some way of es- 
timating it. With respect to the liquid used in tempering, there 



206 Magnetism. 

is no difficulty ; the inquiry relates to the metal, whose heat far 
exceeds the range of our thermometers. The common practice 
is to make use of the colour acquired by the metal, as an indi- 
cation of its temperature ; and we say that it is tempered red- 
white, red, or cherry-red, according to the tint acquired at the 
moment it is plunged into the liquid which is employed to cool it. 
Although this method is necessarily very imperfect, it is still 
sufficient, in most cases, for magnetic experiments, in which dif- 
ferent degrees of temper have no sensible influence, except to a 
certain degree of temperature. The higher degrees do not change 
at all the intensity of the magnetism which bars are capable 
of acquiring, at least when they are magnetized by the processes 
hitherto discovered. This will be shown hereafter when the 
means of measuring this intensity are made known. 



General Considerations respecting the Developement af Magnetism. 
Resemblance to the Electric Pile. 



1 79. The phenomena we have described have so striking a 
resemblance to those of the tourmaline and insulated electric 
pile, that similar theories, it would seem, ought to be applied 
to both. Of this we shall be more and more convinced by a 
stricter comparison. 

In the first place, we recognise two distinct magnetic princi- 
ples, of which each repels that of the same kind and attracts 
that of the opposite kind. These two principles exist originally 
in every bar of iron before it is magnetized, for there is no trans- 
fusion of magnetic principles in the communication of magnetism, 
and nothing either enters into the iron or goes out of it by con- 
tact. The two principles are therefore combined together, and 
each is disguised by the other like the natural electricities of 
bodies, for which reason they exert no action at a distance. 
This action, however, becomes sensible, when they are separat- 
ed by any external influence which acts unequally upon the 
two, in the same manner as the natural electricities of bodies 
manifest their attractive and repulsive properties when they 
have been separated by the influence of an electrified body. 



Developement of Magnetism. 207 

These magnetic principles exist in this manner, and are thus 
developed separately in each particle of iron, without any trans- 
mission of magnetism from one particle to the other. For if a 
magnetic bar is broken into two or three, or any number of 
pieces, each of these pieces exhibits spontaneously two poles, 
like the fragment of a tourmaline, or the elements of an electric- 
al pile, and the poles of opposite names are formed at the ends 
©f the particles which were previously in contact, in the same 
manner as happens in the tourmaline and in the pile. The act 
of separation, however, into several fragments, cannot have any 
influence in producing these poles; it only has the effect of 
displaying them, by withdrawing them from the attraction of the 
contiguous particles by which they were disguised in the entire 
magnetic column, in the same manner as the contiguous electric- 
al poles are exhibi f ed in the fragments of a tourmaline, or the 
elements of a pile. If we would establish this synthetically, we 
have only to join by their extremities several small bars of steel, 
and to magnetize them exactly as if they were one bar, either 
by placing the two extremities of the chain in contact with the 
opposite poles of two magnets, or moving over all its length 
one of the poles of a single magnet. Whatever be the method 
employed, the series of bars will be magnetized in the same 
manner as a continuous bar of the same dimensions, to which 
magnetism has been communicated in a similar manner. If w r e 
employ, for example, short pieces of tempered steel wire, about 
one twelfth of an inch in diameter, and which together make a 
length of about ten inches, we shall find in general that one 
end of the chain exerts the boreal magnetism, and the other 
end the austral magnetism ; but if we break the chain, each 
piece of wire, disengaged from the influence of the other, 
will instantly exhibit two poles, and exert boreal magnetism in 
one half of its length, and austral magnetism in the other half.* 
If we now conceive the dimensions of these little wires diminish- 



* A convenient way of performing this experiment, is to 
place the small pieces of steel wire in contact with one another, in 
a rectilineal groove cut in a piece of wood, and to fix them there 
with wax, in order that they may not separate when they are in the 
act of being magnetized. 



208 Magnetism. 

ed till they are reduced to a simple particle, we shall have an 
exact representation of the state of the particles of iron in mag- 
netic bars, and we may then easily conceive how the system of 
all these little forces may, according to the proportions of those 
which follow one another, give opposite results at the two ex- 
tremities of the bar, or even several results, alternately opposite 
in different points, of its length. 

180. When the two magnetisms have been separated, in the 
particles of a piece of hard iron or steel, experience proves that 
they unite with great slowness. It is necessary, therefore, that 
some cause, existing in the metal and peculiar to its substance, 
should oppose itself to that mutual action by which they have a 
tendency to unite. This cause, whatever it may be, is called 
the coercive force, and may be compared with strict analogy to 
the resistance which electricity meets with in moving along the 
surface, and in the interior, of resinous bodies. The stronger it 
is, the more difficult will it be to communicate the magnetic state, 
and the more durable and constant will this state be, as is the 
case with very hard steel. If, on the contrary, there were no 
resistance, the two magnetisms would separate in each particle 
by the smallest influence, and would re-unite as soon as that in- 
fluence is withdrawn. This is the case with iron, cobalt, and 
nickel, when they have perfect softness. But even in this case 
no transmission of magnetism takes place between one particle 
and another. The composition and decomposition take place 
in the interior of each particle, and between the one and the 
other, there is an absolute impermeability. This is preciselj r 
what happens in the electric pile, formed by plates of glass, 
armed with metal. The decomposition and recomposition of the 
natural electricities are carried on with perfect facility between 
the metallic surfaces, which communicate with each other, with- 
out transmitting any thing through the insulating plates which 
separate them from the rest of the chain. 

The observations which have now been made, appear to give 
a clear and precise view of the intimate constitution of natural 
and artificial magnets. It therefore remains for us only to de- 
termine by experiment the nature and the quantity of free mag- 
netism in each part of the body, and the law which each species 
of magnetism follows in its attraction and repulsion. This sec- 



Developement of Magnetism. 209 

ond point, which can be directly ascertained in electrical ex- 
periments, cannot be here treated in the same manner ; for, as 
we are not able to insulate one of the two magnetisms, we are 
obliged to study the compound phenomena which result from 
their co-existence in those bodies where their distribution is 
known. 

181. If we attend to the distribution of electricity in a state of 
equilibrium in conducting bodies, we shall sec that it is subject 
to a single condition, namely, that all the quantities of electricity 
which are free in the system, shall exercise no attractive or re- 
pulsive force upon any point in the interior of these bodies. In 
magnetism, it is not necessary to an equilibrium that there 
should be no interior action. It is only necessary that it be in- 
ferior to the resistance which the coercive force of the metal 
opposes to the separation or the re-union of the natural magnet- 
isms. But this may take place in a great variety of ways, and 
even with interruptions in the developement of the magnetism in 
the different points of the length of a bar ; so that, in this gen- 
eral point of view, the question is absolutely indeterminate. 

There is one particular case, however, which deserves to be 
considered, chiefly because it presents the limit of all possible 
cases, and is at the same time the most useful of them all, name- 
ly, where the quantity of free magnetism is such, that the sum of 
all the attractive and repulsive forces which result from it for each 
point of the bar, is precisely equal to the resistance which the 
coercive force opposes to the re-union of the natural magnetisms. 
When a bar is in this state, it is evident that it has in each of 
its points, the greatest quantity of free magnetism that it can ad- 
mit ; and hence it is said to be magnetized to saturation. 

The most simple and infallible means of magnetizing to 
saturation, is to subject the bar of steel to so great a magnetic 
influence, that it shall produce instantaneously, in its particles, a 
more powerful decomposition of its natural magnetism, than that 
which can be maintained by the mere resistance of the coercive 
force. For, by withdrawing it from that influence, the first lim- 
it to the re-union of the decomposed magnetisms which presents 
itself, will be that which constitutes the state of being magnetiz- 
ed to saturation. 

E. b M. 27 



210 Magnetism, 

Before we proceed to establish this principle by experiment, 
we must demonstrate the precise law according to which the 
the terrestrial magnet acts upon bars freely suspended; for as 
this action may be exerted at once, without any sensible dimi- 
nution, upon all those that are presented to it, it holds out an 
excellent method of appreciating the intensity of the magnetism 
which we shall have developed. 



Determination and Measure of the Directive Force exerted by the 
Terrestrial Globe upon Magnetized Needles. 

182. When a magnetic needle, freely suspended by its centre 
of gravity, is carried in succession to different places not very re- 
mote from each other, compared with the dimensions of the ter- 
restrial globe, the directions which it assumes, in consequence of 
the magnetic action of this globe are sensibly parallel; and it is 
only by carrying it to places at a great distance from each other 
that we begin to discover some slight deviation from this par- 
allelism. The same result is obtained when we carry it to 
different heights above the surface of the earth, or descend with 
it into deep cavities, provided always that it is kept at a distance 
from minerals, or bodies that have a magnetical action. This 
parallelism, which takes place in the smallest as well as in the 
largest needles, proves that the magnetic force of the terrestrial 
globe may, like that of gravity, be supposed to act in parallel 
lines, with respect to places at a little distance from each other. 
All the mechanical considerations, therefore, by which we cal- 
culate the equilibrium of heavy bodies, may be applied also to 
magnetic bodies, it being supposed that they are heavy and 
magnetic at the same time. 

In order to examine the consequences which result from this 
Fig. 85. principle, let ab be a magnetic needle, of any form, suspended 
by its centre of gravity C, so that it can turn only about this 
point. The action of gravity will be destroyed by the resist- 
ance of the point of suspension ; and, therefore, we may consider 
the needle as destitute of weight, and as influenced only by the 
magnetic forces of the terrestial globe. 



Directive Force of the Earth's Magnetism* 21 1 

In order to analyse distinctly the effects which it will expe- 
rience, we shall take the course we pursued with respect to 
gravity, decomposing, in imagination, the mass of the needle into 
elements so small, that the magnetic state may be reckoned uni- 
form in each, while it varies from one element to another ; and 
selecting at pleasure one of the elements, such as M, we shall 
suppose that it has a certain quantity of free austral magnetism, 
and then determine the forces by which it is urged. This por- 
tion will obviously be attracted by the boreal, and repelled by 
the austral forces of the earth. Let MB be the direction of the 
resultant of the first of these forces, and MA the direction of the 
resultant of the second, and let these lines represent the effect 
of each, when they act upon a certain quantity of boreal or aus- 
tral magnetism, which we shall take as the unity of the magnetic 
mass. Then, by completing the parallelogram MARB, we 
shall combine the two partial resultants into a single one MR, 
equivalent to their united action; and the point M may be con- 
sidered as acted upon by this single resulting force, with an in- 
tensity proportional to the magnetic mass which it possesses; in 
the same manner as the different elements of a heavy body are 
acted upon by gravity in proportion to their density. In our 
climate, where the boreal force is predominant, the resultant 
MR will attract towards the earth the elements which are charg- 
ed with free austral magnetism, and will repel those which are* 
charged with the boreal magnetism. If we, therefore, > q e#gnate 
this, in the first case, by MR, we must, in the second, represent 
it by a line MR', equal and parallel to MR, but having an op- 
posite direction. 

But since all the points of the needle are acted upon by one 
or other of these forces, with an intensity proportional to their 
magnetic mass, it follows, that we may apply here what is de- 
monstrated in mechanics, respecting the equilibrium of systems of 
an invariable form, acted upon by parallel forces. The case is 
indeed absolutely the same as that of heavy bodies of a variable 
density, on the supposition that they have two different weights, 
the one attractive for certain points, and the other repulsive for 
the other points. Hence we derive immediately several impor- 
tant consequences, which we need only enunciate, in order to 
comprehend the truth of them from analogy. 



2 1 2 Magnetism, 

183 (1 .) The attractive forces being each multiplied by the aus- 
tral magnetic mass of the element to which it is applied, will com- 
pose a single resultant GjY, equal to their sum, parallel to their 
common direction, and passing through a certain point G of the 
needle, which will be, relatively to these forces, what the centre- 
of gravity is relatively to gravitation. 

(2.) The repulsive forces also being multiplied by the boreal 
magnetic mass of the elements upon which they act, will like- 
wise compose a single resultant G'S, equal to their sum, parallel to 
their direction, and consequently to that of the attractive forces. 
This resultant will also have its particular centre of application 
G x , whose position depending on the mode of distribution of the 
combined f ;ces, must in general be different from G. 

(3). The magnetic state of the needle being produced solely 
by th^ developement of its natural magnetisms, without any loss or 
any addition, and the quantities of these magnetisms being such, 
that thf ir efforts neutralize each other at each point before their 
separation, it follows that the same equality must still subsist in 
their sum in their magnetic state, which is only a different distri- 
bution of the same efforts. Thus, the total attractive resultant GN 
must be equal to the total repulsive resultant G'S, and their 
sum will be nothing, so that they cannot give to the needle any * 
motion of translation in space. But if we suppose a plane NG 
G'S passing through them, they will tend to make the needle re- 
volve Around the point C, situated in the middle of the straight 
line GG% which joins their two points of application ; and they 
will produce this effect, at lea^t if the straight line GG' is not at 
first placed according to this same direction, as in figure 86 ; 
in this case, there would be no tendency to make the needle turn 
from it. This, therefore, is the position which we must give to 
the magnetic needle round its point of suspension C, in order 
that it may remain in equilibrium. In this case, the vertical 
plane, drawn parallel to the direction SN of the magneti- forces, 
is called the magnetic meridian of the place ; and the straight 
line GG', is called ttr magnetic axis of the needle. When this 
Fig 87. can be compared to a simple rectilineal wire, it always coin- 
cides with the direction of its length, and consequently passes 
through its centre of gravity. 



Directive Force of the Earth? s Magnetism* 213 

(4.) Since the forces GJV, G'S, cannot produce any motion 
of translation, the needle need not be protected against them, Fi g- 85 - 
but only against the vertical effects of gravity. It will 
be sufficient, therefore, for this part of its equilibrium, that 
its centre of gravity be supported vertically ; as, for example, 
by a vertical wire incapable of extension, whose upper end 
is attached to a fixed point. The vertically of this wire 
will in no respect be disturbed by the magnetic forces. To show 
the truth of this position in a rigorous manner, let us suppose the 
total resultant G"R", of the magnetic forces, if it is not zero, to ' 8 ' 
be decomposed into two forces, one of which, G"V", is vertical, 
and the other, G"H", horizontal, and directed in the magnetic 
meridian*. If the first of these is not zero, it must either act 
in conjunction with, or in opposition to, the force of gravity, 
and consequently increase or diminish the weight of the 
needle. But this does not happen ; for the weight of the needle, 
determined by the nicest balances, is the same after it is 
magnetized as before. In order to try now the horizon- 
tal force, suspend to an untwisted silk fibre CZ, a strip of Fi S- 8i> - 
card AB, and upon one of its extremities perpendicular to its 
direction, adjust a magnetic needle a b, balancing it by a 
weight M at the opposite end, so that the card may be hori- 
zontal. Let the system be then turned, so that a b may be 
exactly in the magnetic meridian, as determined by observa- 
tion with another magnetic needle, suspended horizontally from 
its centre of gravity, and AB will be found to remain in 
equilibrium in this position. But this could not take place, if 
the horizontal magnetic force, directed in the line a b, were 
not absolutely zero, since, otherwise, it would have tended 
to make the lever CA turn round its point of suspension C. This 
force is, therefore, zero, as well as the vertical force, and 
consequently the total resultant is also zero, as has been in- 
dicated by mechanical considerations, founded on the mode in 
which magnetism is developed, a mode which is thus established 
in a rigorous manner. 

1 84. Being now acquainted with the manner in which the mag- 
netic forces of the earth are combined in acting upon a magnetic 
needle, suspended by its centre of gravity, we are able to cal- 
culate the effect of these forces upon a needle iii all the positions 



214 Magnetism. 

in which it can be placed, and thence to determine the law of 
the motions thus produced. 

Fig. 90. For this purpose, let us conceive the attractive resultant GN, 
and the repulsive resultant G'aS, decomposed each into two others, 
namely, one vertical GV, G'V, and the other horizontal GF, 
G'JBP, and directed in the magnetic meridian ; aftd let us call 
V, V\ the vertical forces, resulting from this decomposition, and 
H, H', the horizontal forces. Then let us examine separately 
the efforts which each of these two systems is capable of pro- 
ducing, beginning with the horizontal forces H, H'. 

Fig. 91. Let us take a magnetic needle a 6, in which the direc- 
tion GG / of its magnetic axis is supposed to be known, as we 
shall presently see how it is to be found. Let this needle 
be suspended by its centre of gravity C, with an assemblage 
of- untwisted silk fibres, and let it be balanctd by a small 
weight, placed upon the south branch, so that the axis GG / may 
be horizontal. The effect of the vertical forces F, P, will then 
be destroyed, and there will remain only the horizontal forces 
H, H', the effect of which will tend to draw the axis GG' into 
the direction of the magnetic meridian, to which they are paral- 
lel, so that they will not impress upon it any motion, if it is al- 
ready in that direction ; but, however small be its deviation, it will 
be drawn into this position by a series of oscillations, in the same 
manner as a pendulum, pushed from the vertical, is made to os- 
cillate on each side of that line by the action of gravity. And 
as, in this last case, allowance being made for the resistance of 
the air, the excursions are the same on both sides of the vertical, 
so also, allowing for the resistance in question, and supposing the 
torsion thread deprived of its sensible elastic re-action, the mag- 
netic meridian must pass through the middle points of the arcs 
described by the needle. This circumstance, therefore, will 
enable us to ascertain the magnetic meridian during the oscilla- 
tions themselves, which is particularly important at sea, as there 
the needle is always agitated ; whereas, on land, the resistance 
of the air gradually destroying its motions, it will become sta- 
tionary of itself. When it has taken its position, we measure the 
angle comprehended between its direction and that of the celes- 
tial meridian. In this way we are able to find the magnetic me- 
ridian MM', and the declination of the needle for the place where 



Directive Force of the Earth's Magnetism. 215 

the observation is made. It has already been remarked, that thjs 
declination differs in different places, and is subject to sensible 
variations in the same place ; but deferring this subject for the 
present, we shall inquire into the conditions of equilibrium of the 
needle, on the supposition that the magnetic force is constant. 
1 85. The energy with which the needle is drawn into the mag- 
netic meridian, depends on the absolute intensity of the attractive 
and repulsive forces GH, G'H', by which it is affected ; but it 
depends also on the position of their points of application G, G' ; 
for the magnetic axis may be compared to a lever urged si- 
multaneously by these two forces. If the points of application 
G, G', for example, are both on the same side of the vertical 
axis of suspension, it is obvious that the rotatory action of the 
two forces will oppose each other, and the resultant action will 
be equal to the difference of their statical moments, calculated 
in relation to this axis. On the other hand, if the points of ap- 
plication G, G', are situated on different sides of the axis of sus- 
pension, the rotatory forces will conspire, and produce an effect 
equal to the sum of their statical moments. This disposition 
will, then, be much more advantageous for overcoming the fric- 
tion, and inertia of the suspension, when the needle is supported 
upon pivots ; and therefore we must seek the means of obtaining 
such a distribution of magnetism. We shall soon do this ; and we 
shall see, that, by a method called the double touch, this state can 
be produced in needles whose composition is homogeneous, and 
whose form about their centre of gravity is symmetrical. Then 
the points Gfi'. may be obtained at equal distances on each side 
of the centre of gravity Cof the needle, so that the axis GG' may 
pass through this centre. The rotatory forces will then become 
equal, and their effects will be exactly the same as if the needle 
were influenced by a single force, double of the preceding, and 
applied at one of the two points G, G', and on one side of the 
axis of suspension. It is obvious from this reasoning, that, in 
general, the points of application G, G', as well as the directions 
and intensities of the horizontal directive forces //, H\ are fixed 
for the same nemlle, whatever be its position in the piano of the 
horizon, provide we do not alter its magnetic state, and it re- 
mains in the same place. Hence it is evident, that if we push it 
ever so little from the magnetic meridian, the efforts of the hor- 



216 Magnetism. 

izpntal forces H, H', to draw it back, will be precisely the same 
as in the case of a pendulum, that is, will be proportional to the 
' sine of the angle of deviation. Let MM' be the direction of the 
horizontal magnetic meridian, and let us suppose the needle 
a b, previously balanced, and at rest, in this line, to be turn- 
ed from it by any force into the direction .AW. Then the two 
horizontal directive forces GH, G'H, applied at the points G, G', 
fixed in the needle, continuing to act parallel to the magnetic 
meridian MM, will tend to draw back the needle into this line. 
In order to determine the measure of their efforts, we have only 
to represent their total intensity by the lines GH, G'H, them- 
selves, and then letting fall irom the extremities H, H, the per- 
pendiculars HQ* H'Q', upon the direction of the needle prolonged, 
the rotatory forces, according to the rule of the parallelogram of 
forces, will be represented by these perpendiculars ; that is, they 
will be respectively equal to the products obtained by multiply- 
ing GH and G'H by the sines of the angles NGH, NGH', 
which measure the deviations of the needle from the magnetic 
meridian. 

186. Coulomb has verified these results by means of the tor- 
sion balance. This apparatus, represented in figure 92, consists 
principally of a wire, generally of silver or copper, drawn 
into a very slender fibre, at the bottom of which is suspen- 
ded a needle in a horizontal position. The whole is in- 
closed in a glass case. A graduated circle Z, placed at the 
upper point of attachment, enables us to turn it through any re- 
quired number of degrees. If the suspended needle were not 
retained by any force, it is obvious that it would obey the motion 
of the wire, and, after some oscillations, it would be found to 
have turned horizontally the exact number of degrees marked 
by the graduated circle ; but as it is drawn at the same time by 
the directive forces GH, G'H', which tend to bring it back to the 
magnetic meridian, it must twist the wire in order to return, and 
twist it to such a degree, as to oppose a resistance equal to the 
rotatory action of the components GH, G'H', in the position 
where the needle stops. If, when this equilibrium is established, 
we turn the wire again in the same direction's before, and 
through a different number of degrees, indicated by the graduat- 
ed circle, the needle will again change its place, and will settle 



Fig. 91. 



Directive Force of the Earth' ] s Magnetism. 217 

in a new position of equilibrium, in which the moments of 
the rotatory forces GP G'P', will be still equal to the re-action 
produced by the total torsion which the wire has experienced. 
By repeating the experiment in this manner for several devia- 
tions of the needle, we shall have as many corresponding tor- 
sions of the wire which produce the equilibrium. But it may 
be demonstrated by experiment, and also by the theory of elas- 
tic re-actions, that, in general, while the wire is not over-twist- 
ed the re-action of torsion is proportional to the arc of torsion 
which it has received ; therefore the observation of these arcs 
will afford a measure of the directive forces for different devia- 
tions of the needle from the magnetic meridian; and by a 
comparison of these measures, it will be easy to determine if the 
forces in question are really proportional to the sine of the devi- 
ations, as the theory of the composition of forces has indicated. 
In order to do this in a convenient manner, Coulomb adapt- 
ed to the lower end of the suspending wire, a small stirrup EE, Fig. 93. 
formed of a very light plate of copper. This stirrup serves to 
hold the needle ; and in order that the needle may always be 
placed in the same position, the surface EE of the plate is 
covered within with sealing wax, upon which the impression 
of the needle is made when the wax is in a soft state. Be- 
fore using this apparatus, a needle of copper, or of any other 
metal that is not magnetic, is placed in the stirrup, and the gradu- 
ated circle is turned to such a degree, that when the needle is at 
rest, it shall direct itself to the zero of the lateral scale, engraved 
upon the side of the glass case. When this is done, the whole 
apparatus is turned, till the zero and the copper needle are ex- 
actly in the direction of the magnetic meridian, previously de- • 
termined by observation ; a magnetic needle is then substi- 
tuted in the place of the copper one. The suspending wire is 
now twisted through different angles, and the deviations of the 
needle from the magnetic meridian are observed. The oscilla- 
tions which precede the settlement of the needle, have a dura- 
tion proportional to its mass, compared with that of the air ; but 
in order to stop them sooner, there is fixed under the stirrup, 
by a small rod of copper, a very thin vertical plate of copper 
VV, which is immersed entirely in a vessel of water. The 
resistance of the fluid deadens the oscillations very quickly, and 
E. <fr M. 27 



218 



Magnetism, 



without preventing the needle from arriving at the degree of 
deviation which corresponds to the torsion of the wire. It is 
necessary, however, to take great care that the plate VV be en- 
tirely immersed, in order to get rid of the effect of the aqueous 
ring which is raised round it by capillary attraction ; and in 
order to give precision to the results, the size and elasticity of 
the suspending wire must be suited to the magnetic force of the 
needle, employing large wires for stronger needles, and reserv- 
ing the very fine wires for the case, where the directive force is 
very feeble. 

In this way Coulomb obtained the following results, with a 
magnetic needle 22 inches long, and a line and a half in diame- 
ter. The suspending wire was of copper, of the size called No. 
1 2 in commerce ; it was six feet long, and weighed five grains. 



Circles of Torsion 


Angles of Deviation 


Force of Torsion 


given by the grad- 


at which the Needle 


in Degrees. 


uated circle. 


comes to rest. 




0° 


0° 


0° 


1 


m 


349| 


2 


21£ 


698| 


3 


33 


1047 


4 


46 


1394 


5 


631 


1736| 


H 


85 


1895 



From these results, it appears that the first forces of torsion 
are proportional to the deviation of the needle. This conclu- 
sion, indeed, is a necessary one ; for, since the total directive 
force is proportional to the sine of the deviation, it ought in 
small angles to be sensibly proportional to this deviation. In 
order to verify this value in the greatest angles, we have only 
to divide each torsion by the sine of the corresponding angle of 
deviation, and see if the quotient of this division is a constant 
quantity. If we make trial of this method with respect to any 
of the preceding observations, taken at pleasure, we shall find 
the following results. 



Directive Force of the Earth's Magnetism 



219 



Observed Deviation. 


Result of the Division. 


10°1 


1917,85 


21i 


1927,92 


33 


1922,37 


46 


1937,89 


631 


1940,37 


85 


1902,24 


Mean....l 924,77 



The agreement of these results shows the truth of the law. 
It follows that the constant quantity 1924,77 expresses the 
force of torsion necessary to retain the needle at 90° from the 
magnetic meridian, because the sine of 90 is equal to unity. 
Now if we divide the observed torsions by this constant quantity, 
we shall have the sines of the other angles, where the needle 
must stop in each experiment, and can compare them with the 
results of observation, as in the following table. 



Observed Tor- 
sions. 


Deviations. 


Oiflerences between 

the Calculated and 

Observed Results. 


Observed. 


Calculated. 


o 
349,50 
698,75 
1047,00 
1394,00 
1736,50 
1895,00 


10° 30' 
21 15 
33 
46 
63 30 
85 


10° 28' 
21 17 
32 57 
46 24 
64 27 
79 55 


— 0o 2' 
+0 2 
—0 3 
-|-0 24 
-j-0 57 
—5 5 



The last of these differences, which is the only one that de- 
serves notice, depends probably on a small alteration produced 
in the re-action of the wire, by the great degree of torsion to 
which it was necessarily subjected in the last experiment. The 
perfect agreement of all the other results confirms the accuracy 
of the law. 

This law is one of great importance and constant utility, 
since it enables us in all experiments to calculate the horizontal 
influence exerted on the magnetic needle by the terrestrial globe, 
the action of which must necessarily be combined with all the 
other forces that affect it. 



220 Magnetism. 

181. We pass now to the examination of the effects produced 
by the vertical components F, V . These may be easily calcu- 
lated by means of the dipping needle, which is shewn in figure 
80, and of which we have already given some account. In this 
instrument the needle can only turn vertically round its horizon- 
tal axis, without going out of the vertical plane, where we place 
the circle in which it oscillates. Let us suppose, therefore, that 
this circle is directed perpendicularly to the magnetic meridian, 
Fig. 94. and then the horizontal forces H, H', which are parallel to this 
plane, will be completely destroyed by the pivots of the suspen- 
sion ; the vertical forces, J 7 , P, therefore, acting alone, will tend 
to turn the magnetic axis in their own direction, that is to say, 
to make it vertical, so that if we place it in this position, it will 
remain at rest. Here, then, we have a mark for recognising the 
direction perpendicular to the magnetic meridian ; we have only 
to turn the vertical circle, which carries the needle till this con- 
dition is fulfilled, and having noted on the horizontal division 
where it stops, to bring it back to 90° from this point, and it will 
be found in the magnetic meridian. 

In this new position, if we observe on the vertical circle the 
point of the division where the magnetic axis stops, the arc com- 
prehended between this division and the lower vertical point of 
the same circle will give the magnetic inclination, reckoned from 
the vertical of the place. In order that this observation and the 
preceding may be exact, great care must be taken to verify the 
horizontal position of the azimuth circle on which the instru- 
ment rests, and which for this purpose is furnished with two lev- 
els placed at right angles to each other. 

The dipping needle being directed in the magnetic meridian, 
and its magnetic axis being in equilibrium upon the direction of 
the forces which influence it, push it ever so little out of the di- 
rection of its equilibrium, without making it deviate from the 
same vertical plane, it is manifest that the magnetic forces will 
tend to bring it back by a series of oscillations, the laws of 
which will be absolutely the same as those of a pendulum oscil- 
lating by the action of gravity. 

Without knowing the distribution of magnetism in each point 
of the needle, we conceive that the rapidity of its oscillations 
will depend upon this distribution, upon the absolute quantity of 



Directive Force of the Earth's Magnetism. 221 

free magnetism in each of its points, and lastly upon the en- 
ergy with which the resultant of the terrestrial forces acts upon 
this magnetism. The two first elements will be constant for the 
same needle, if it preserve always the same magnetic state, or if, 
having lost it, it is remagnetized in the same manner and to the 
same degree. The third only, namely, the terrestrial force will 
vary by change of place ; and in the same manner as in the 
case of the pendulum, the duration of the oscillations being re- 
ciprocally proportional to the square roots of the forces. Hence 
if we carry this needle to different parts of the earth, and count 
the number of oscillations which it there perforins in the mag- 
netic meridian, in the same number of seconds, we may without 
any calculation compare the intensities of the magnetic force of 
the globe at these different places, in the same manner as we 
compare the intensities of gravity by means of the pendulum, 
and in both these cases, the intensities will be proportional to 
the squares of the number of oscillations. 

M. De Humboldt, for example, having carried the same dip- 
ping needle from Paris to Peru, and from Peru to Paris, found 
that before his departure and after his return, it performed at 
Paris 245 oscillations in ten minutes of time, whereas at Peru 
it performed only 211. The intensity of the magnetic force, 
therefore, at Paris, is to its intensity at Peru, as the square of M 
245 to that of 21 1 ; that is, as 60025 is to 44520, or as 135 is to 248. 
100. We shall soon inquire into the laws of these variations. 

As the method of observing which we have just explained is 
liable always to some inaccuracy, owing to friction, we may ad- 
vantageously substitute the observation of the horizontal oscilla- 
tions, combined with that of the magnetic inclination. Suppose 
that we have counted the oscillations of a horizontal needle, 
suspended by an assemblage of silk fibres, which may be 
considered as having no torsion, it will be easy to deduce from 
this the number of oscillations that the same needle would 
have performed round the direction of the magnetic inclination, 
if it had been freely suspended ; for the horizontal forces H, H', Fig. 9©. 
are simply the total forces GJV, G£, decomposed horizontally 
in the plane of the magnetic meridian, and therefore we can de- 
duce these last by the rule of the parallelogram of forces, when 
ihe inclination of the one to the other is known. According to 



222 Magnetism. 



&• 



what is here laid down, the number of oscillations produced in 
the same time by these two kinds of forces will be proportional 
to the square roots of their intensities. Consequently, if we ob- 
serve the horizontal oscillations of the same needle in different 
parts of the earth, and determine at the same time by direct ob- 
servation the inclination in each of these places, we can thus 
ascertain the intensities of the absolute corresponding forces 
with more exactness than with the dipping needle. But we must 
not bring the needle to a horizontal position by a counterpoise, 
since this being different in different latitudes, would affect the 
statical moment of the mass to be moved. We must give it this 
direction by placing it in a small paper dish attached to the 
bottom of the suspending wire, the stiffness of which will be 
sufficient to prevent it from inclining, as represented in figure 95. 
The weight of this dish being essentially nothing, and being 
very nearly in the axis of suspension, will not sensibly affect the 
duration of the oscillations, and consequently it may be contin- 
ued or renewed, and the observations may still be compared 
with each other. 

This method of horizontal oscillations may also be made use 
of to compare, in the same place, the energies of the directive 
forces of the same needle or of several equal needles, magnetized 
in different ways. For these intensities are to each other, as 
the squares of the number of oscillations made in equal times. 
This process is one of very frequent use. 

Having by the preceding methods fully described the pro- 
cesses used in determining the peculiar action exerted by the 
horizontal and vertical elements that compose the magnetic 
force of the globe, when these elements act separately ; we may 
by combining these actions according to the calculus, discover 
the motions that will be imparted to a magnetic needle directed 
to any point of space whatever. But as an exact observation of 
these combined motions would be attended with very consider- 
able difficulty, it is generally avoided by experimentalists. And 
they chiefly confine themselves to measuring those partial effects 
which we have described. It is evident that these are sufficient 
for determining in any place the elements of terrestrial magnet- 
ism, that is, the declination, inclination, and intensity of the 
magnetic resultant. 



Directive Force of 4he Earth's Magnetism. 223. 

Hitherto our observations on the direction of needles have 
related to the magnetic axis. We must, therefore, know how 
to determine this axis, which is done in the following man- 
ner. 

Let ABCD represent a magnetic needle, suspended horizon- F >g- 9Q - 
tally by a number of untwisted silk fibres, either immediately or 
by means of a small paper or copper dish. When this needle 
is in equilibrium, its magnetic axis GG / will be directed in the 
magnetic meridian of the place. This, however, is not sufficient 
for determining it, since its position in the needle is only ideal; 
but, from the nature of parallel forces, we know that, whatever 
position be given to the needle, this axis must remain fixed, and 
preserve invariably the same situation relatively to the surface by 
which it is bounded. Having now observed to what terrestrial 
object one of the sides AB is directed when it is in equilibrium, 
we then turn it upside down, and suspend it anew horizontally 
by the same kind of suspension as in the first experiment. The 
magnetic axis GG' will again place itself in the direction of the 
magnetic meridian ; but the sides of the needle having turned 
circularly round this axis, will not again place themselves in the 
same direction as before ; and, what is a point of great importance, 
they will deviate from the magnetic meridian as much as they 
did before, but in an opposite direction. This is shown in the 
figure where the dotted line A'B'QD' represents the position of 
the surfaces after their reversion. Having observed the direc- 
tion of one of the sides AB of the needle in its first position, if 
we do the same for the second, the true direction of the magnet- 
ic meridian will be exactly midway between them. We may 
thus determine and note it on the surface of the needle, or mark 
the point of space to which it corresponds when prolonged. 

In order to ascertain the true value of the magnetic inclina- 
tion, we must perform a similar experiment with the dipping 
needle. Suppose such a needle exactly suspended by its centre 
of gravity C, and observe the point of the vertical circle where Fig. 97. 
it stops, when one of its faces E is turned to the east from the 
magnetic meridian, then the magnetic axis GG' will be found 
in the direction of the two resultants GJV, G'S, and the angle Fig 86. 
which this axis forms with the vertical, will be the inclination 
sought. But as the ideal line GG cannot be observed, let the 



224 Magnetism. 

instrument be shifted so that the face of the needle, before turn- 
ed to the east, shall be now turned to the west, the line GG will 
be invariably found in the same position. Hence, if it is not 
symmetrically directed in relation to any of the rectilineal sides 
of the needle, these will correspond to different points of the 
circular division, and we shall have the true inclination by tak- 
ing a mean of their results. In this reasoning, we have suppos- 
ed that the axis of suspension passes exactly through the centre 
of gravity of the> needle. If this condition is not fulfilled, the 
the inclination deduced by the method just described will be 
inaccurate. We shall presently see how this error may be 
corrected. 

The general form which artists have adopted for the needle, 
is that of an arrow, whether it is used to point out the dip or the 
variation. This form is no doubt more convenient for indicat- 
ing the exact division at which the needle settles, and it has also 
the advantage of giving, with the same weight, a directive force 
perceptibly greater, as we shall show hereafter ; but this does 
not prevent us from practising with these needles, the kind of 
reversion just described ; for it is never certain that the point of 
application of the magnetic resultant is in the axis of the needle, 
that is to say, in the right line which joins its two extreme points ; 
and it is only by the experiment above given, that the true di- 
rection can be determined. 

In observing the inclination, there is still another precaution 
to be observed in order to insure great accuracy. We have hith- 
erto supposed that the needle is exactly suspended by its centre 
of gravity, in consequence of which no attention was paid to the 
action of terrestrial gravity ; in practice, however, it is difficult, 
if not impossible, to fulfil this condition rigorously ; and if it is not 
fulfilled, a considerable inconvenience must arise. 

Since one half of the needle has a greater tendency than the 
other to fall towards the earth, it must descend more than it 
would do by the action of terrestrial magnetism ; and, according 
to the kind of magnetism which this half possesses, the true in- 
clination will be increased or diminished ; and there arises also 
an error in observing the intensity, in consequence of the oscilla- 
tions being no longer performed round the directions GvV, G'S, 
of the resultant of the magnetic force alone. This circumstance 



Different Methods of Magnetizing. 225 

however, suggests the means of correcting the error of centering, 
which is the cause of it, at least when we know that the error 
is very small. For this purpose, we must magnetize the needle 
successively in two opposite directions, so as to invert its poles ; 
and then observe the inclination and intensity of the needle afier 
each of these operations. Care must always be taken to suspend 
it exactly in the same manner and by the same points, a condition 
easily fulfilled by giving an invariable position to the cap and 
the pivots by which it is supported. If the transverse axis ter- 
minated by these pivots does not pass exactly through the centre 
of gravity of the needle, one of the observed inclinations will be 
too great and the other too small, and the mean will give very 
nearly the true value of the inclination and the intensity, at 
least if the two branches of the needle are symmetrical, and the 
distribution of magnetism the same in the two operations. 

The observations which we have now made are applicable 
to all needles, and are independent of the manner in which the 
free magnetism is distributed in them. They suppose only that 
the magnetic state of each needle does not change in the differ- 
ent situations in which it is placed. Now, if we compare the 
intensities of the action exerted by the terrestrial magnet on the 
the needle, magnetized in different ways, we shall be able to 
appreciate the degree of developement produced in the natural 
magnetisms, and to recognize the most advantageous processes 
for effecting it. This will be the object of the following section. 



Of the Different Methods of Magnetizing. 

191. Of all the methods of developing the magnetic forces, 
the most simple is that which we have explained in the first sec- 
tion. It consists in bringing the extremity b of a bar of steel Fig. 
or hard iron within a short distance, or even into contact with the 
north or south pole A of a magnet, AB. The free magnetisms 
in A and B act upon the natural magnetisms of the bar of steel 
a b ; but the pole A being nearest, its power will predominate, 
and the decomposition will be effected in every metallic particle 
of a b. The magnetism of an opposite name to A is attracted; 
E. $r M. 29 



226 Magnetism* 

that of the same name is repelled, and by a series of separations- 
of this kind the extremity b of the bar acquires a pole of ara 
opposite kind to A. 

In order to be convinced of this, we have only to form with 
a steel wire a small needle aft about one-fourth of an inch long y 
and to magnetize it by rubbing it several times, and in the same 
direction, upon the pole A. When this needle, suspended by its 
centre with a single fibre of silk, is brought near the pole A, one 
of its extremities, ft for example, will be attracted, and will turn 
itself towards this pole •, but if the same needle is brought near 
the extremity b of the bar, which has been in contact with A, it 
will immediately whirl round. The extremity P which was 
attracted by A will be repelled by 6, and on the contrary, a will 
be attracted. If we continue to present this needle to different 
points of the bar b a, beginning at the extremity 6, we shall find 
that through a certain length b c, the magnetism is of the same 
kind as at &, but an opposite magnetism will immediately suc- 
ceed, for the needle will turn round and present its other pole to 
the bar. If the bar is short and the magnet powerful, this new 
state will continue without interruption to the extremity a, and 
consequently the bar will have in its second half a c, a magnet- 
ism of the same nature as A, and in its first half b c an opposite 
magnetism. The point c will be in a neutral state. 

When the bar is very long, it often happens that the second 
state does not extend to the extremity, but only to a certain dis- 
Fi o- 99 tance c r . Then the new magnetism which begins to show itself in 
c, exhibits, first, in departing from this point an increasing energy, 
manifested by the rapidity of the motions which it imparts to the 
small needle ; but, afterwards, beyond a certain distance «', this 
energy begins to diminish, and is nothing at the point c', where 
the needle again becomes indifferent. Then succeeds another 
magnetism which is contrary to«#, and to this succeeds sometimes 
even a fourth which is similar to A, and so on. The trial needle 
indicates these alternations by the inversions which it experi- 
ences at every change of magnetism ; and the points of the bar 
a b, where this happens, are called consecutive points. 

If we suspend a bar of this kind for the purpose of determin- 
ing its directive force, it is obvious that the parts situated on the 
same side of the centre of suspension, which have magnetisms of 



Different Methods of Magnetizing. 227 

an opposite kind, will also have opposite tendencies, the one to 
bring the extremity of the bar towards the south pole of the earth, 
and the other towards the north pole. The total directive force 
of the bar will therefore, in general, be more weak than if each of 
its halves possessed throughout its whole length only one kind of 
magnetism. On this account, it is of the greatest importance to 
avoid consecutive points, not only in the formation of compass 
needles, but also in every case ; for a bar will never produce 
the effect which might be obtained from it if these alternations did 
not exist. Whatever, indeed, be the kind of experiment for 
which we employ it, the poles of an opposite name will act 
always at the same time, and their action will be opposed to each 
other in proportion to their proximity, because their distances 
from the points attracted or repelled will then be less different. 
Hence, the most favourable arrangement of magnetism is, when 
only one kind of magnetism exists in each half of the bar ; and, 
therefore, this mode of separation, produced to the greatest ex- 
tent, is the object which should be kept in view in all our re- 
searches. 

192. When we have magnetized a bar a b in the manner 
now supposed, by putting one of its extremities b in contact 
with one of the poles A of a loadstone, the consecutive points 
will be more easily formed, if the metal of the bar is hard 
either by its nature or in consequence of tempering. The rea- 
son of this is evident. The action of the loadstone AB decreases 
with the distance, and there is always a certain point of the bar 
a b where it becomes equal to the coercive force. Consequently, 
all the points situated beyond this limit would not undergo any 
decomposition of their natural magnetisms, if they were subject- 
ed solely to the influence of the loadstone AB ; but the first part 
b c of the bar where the magnetism is already developed, acts 
also upon these points, and tends to develope the opposite mag- 
netism, and therefore the resultant of this action, commencing at 
a shorter distance than that of the loadstone AB, there must be a 
point at which it predominates, and it is there that the first alter- 
nation will take place, and this must occur the nearer to the 
point b according as the coercive force is greater ; for, if it were 
infinite, the magnet AB would only develope magnetism in the 
point b which is in contact with its pole A. The same reasoning 



228 Magnetism. 

is applicable to a comparison of the action exerted upon the rest 
of the Bar by the first alternation b c, and the second c a' c'. 
The predominance of this last over the following points, in con- 
sequence of its proximity, will be so much the more sensible as 
the coercive force is greater, and there will therefore be a greater 
facility in producing a third alternation db' c". From this man- 
ner of viewing the phenomenon, the energy of the successive 
poles a', b', a", b", must diminish gradually in proportion as they 
are removed from the first extremity &, where the magnetism is 
most powerfully developed ; as may be shown experimentally, 
by comparing the weights which adhere to the different parts of 
the bar, or by the oscillations of the trial needle. 

The following experiment will, we trust, remove any difficul- 
ties that may appear to belong to this theory, as it proves that 
the same effects which w r e have described are produced by elec- 
tricity. 

Take a tube of polished glass several feet long, and having 
suspended it by silk threads, touch one of its extremities for some 
time with a stick of sealing wax, excited by friction. Upon ex- 
amining the electrical state of the tube, we shall find that through 
a certain length from its touched extremity, it has the same 
kind of electricity with the wax. To this part there succeeds 
another, which possesses the opposite electricity, but in a weaker 
degree; and beyond this there will be found a third exhibiting 
the same electricity with the wax, but in a still more feeble 
manner. These alternations will continue to the other end of 
the tube, and will be proportioned in their number and extent 
to the force of the electricity which is employed. Here then 
we have the consecutive points of the magnet, with this differ- 
ence only, that the electricity of the wax passes at first upon the 
glass, and extends over a certain length, because neither of these 
bodies resists entirely the direct transmission of electricity; 
whereas the particles of iron are rigorously impermeable to the 
transmission of magnetism. For this reason, the first alternation 
in magnetized bars acquires always a magnetism opposite to that 
of the pole of the loadstone which touches it, while the first alter- 
nation of electricity is of the same nature in the glass as in the 
wax. 



Different Methods of Magnetizing. 229 

193. All the phenomena of the composition and decomposi- 
tion of the two electricities may, in general, be represented by 
the two magnetisms, with the modifications only which arise from 
absolute impermeability. As this analogy is of great import- 
ance in pointing out the truth of the theory, we shall now pre- 
sent some examples of it. 

The first which we shall give is from Dr Gilbert's work. 
Place a loadstone or magnetic bar AB, so that the two poles Fig. 100. 
JL, B, shall be in a vertical position, and taking two small pieces 
of soft iron wire a 6, a'b', of the same length, and about an 
inch long, suspend both of them by untwisted silk fibres, 
s a, s' a', and bring them gradually near the pole A. When 
they are not very distant, as about the eighth of an inch, they 
will avoid one another as if they were mutually repelled, and 
the two suspended wires vvill diverge. The cause of this phe- 
nomenon is very simple. In proportion as the wire, a b, for 
example, approaches the pole A, its natural magnetisms will be 
decomposed by the predominating influence of this pole ; a b will 
therefore become magnetic, and acquire tw r o poles, one of which, 
b, is of an opposite name to A, and the other of the same name. 
The same thing will happen to the second wire a' b'. The ex- 
tremities of these two wires, which are in contact, will therefore 
suddenly acquire magnetisms of the same name, and therefore 
will repel each other ; and this cause, favoured by their small 
size, and the mobility of their suspending fibres, will show itself 
by their divergence. 

Here, then, is an exact representation of the electric influences, 
with this difference only, that there is no real transmission of 
magnetism into the different parts of the wire a 6, but simply a 
decomposition in each particle; a decomposition, in virtue of 
which one of the two kinds of magnetism becomes free in &, 
and the other in a, while the opposite magnetism is disguised. 

Let us now take two plates of steel AB, A'B', of the same Fig. 101. 
length, and very thin, like that which is used for watch springs. 
When both of them are magnetized in the same manner, by 
putting them in contact with the same pole of the same loadstone, 
or by rubbing them in the same direction on its surfaee, let it be 
ascertained what weight either of them, for example, AB, is 
capable of supporting by its pole A, and then suspend from this 



230 Magnetism. 

pole a soft iron wire b a, whose weight is about 50 or 100 times 
less. When this is done, bring the second plate A'B' slowly 
towards the first, or if you please, place the one upon the other, 
with their opposite poles coincident, and when they come in con- 
tact, the adherence of the wire b a will be almost entirely des- 
troyed ; so that the system of two magnets thus combined 
can only support a very small portion of the weight, which 
each of them would have supported separately. This phenom- 
enon is easily understood. For if these two plates, being 
equal in dimensions and magnetic energy, are so placed, that their 
opposite poles act simultaneously on ferruginous particles which 
are exactly, or very nearly, equidistant, it is clear that their 
actions would neutralize each other, as if the united plates 
formed a uniform mass in which the boreal and austral magnet- 
isms were cc-mbined. This phenomenon is entirely analogous to 
41. that heretofore described, respecting the contact of two glass 
plates charged with opposite electricities by mutual friction, and 
it is evident, that the same explanation applies to both. 

We have supposed the two plates to be very thin, in order 
that their distance, at different points, from the wire a 6, may be 
nearly equal, when they are placed upon one another. Indeed, 
in the second plate A'B' this distance is unavoidably greater 
by the whole thickness of the first AB; and it is from this cause- 
that the actions of the two equal plates are not exactly destroy- 
ed. This inequality of action, however, though it cannot be 
removed, may be diminished by diminishing the thickness of 
the plates ; or the same effect may be produced by taking a 
second plate A'B' more powerful than AB. The wire a b will 
then fall of its own accord, when the action of the plate A'B\ 
diminished by the excess of distance, becomes equal to the 
action of AB, which will happen either at the instant of the con- 
tact of the two plates, or before it. In this case, if the distance 
of the two plates is farther diminished, the action of the second 
will finally preponderate, and the wire a b will return again to 
attach itself to the pole A, but with a magnetism opposite to that 
which it had at first. It is easy to vary these phenomena ; but 
the two experiments which we have described will suggest the 
explanation to be given in all other cases. 



Different Methods of Magnetizing. 231 

194. We have already remarked, that a loadstone loses noth- 
ing by being employed to magnetize any number of bars ; on the 
contrary it will be seen, that the effect of these repeated opera- 
tions, instead of diminishing, rather increases its energy. When 
the pole of a loadstone touches the extremity 6 of a bar, and Fig. 98. 
developes in it a magnetism contrary to that which it possesses, 
this magnetism, in its turn, acts upon the natural magnetism of 
the loadstone which produces it, and tends to excite in it a new 
decomposition, which augments the free magnetism of A. This 
augmentation produces in the bar a b a new decomposition, 
which again re-acts upon the pole A, so that both of them, by 
this mutual re-action, acquire a more intense degree of magnet- 
ism than they would have done by the direct action of A. This 
is perfectly analogous to the increase of charge which the upper 
plate of an electrical condenser receives from the action of the 
electricity which is disguised in the lower plate ; but the electric 
equilibrium establishes itself instantaneously in the plates, be- 
cause they are composed of materials capable of transmitting 
electricity with extreme facility; whereas the maximum charge 
of a loadstone, and of the bars which touch it, is produced 
slowly. For, on the one hand, if these bars are made of steel 
or hard iron, the coercive force opposes itself to a ready decom- 
position of their natural magnetisms ; and on the other hand, the 
substance of the loadstone opposes to the increase of its magnet- 
ism a similar resistance. The first of these obstacles may be 
destroyed by making the bars of very soft iron, but the second 
is unavoidable; and it follows from this, that it must require c & 
good deal of time, for the system to develope all the magnetism 
which it is capable of acquiring. 

This remark will serve to explain several important phenom- 
ena. Suppose a small piece of soft iron to be applied to one of 
the poles of a natural or artificial magnet, and from this iron <* 
small balance scale to be suspended, in which are placed succes- 
sively different weights. If at first we put into this scale the 
greatest weight it is capable of supporting, it will be found 
that this weight may be increased by a very small quantity 
every day ; but if, at the end of some weeks, or even months, 
we forcibly detach all the iron, and try again to replace it, 
we shall find that the magnet is no longer capable of support- 



232 Magnetism. 

ing it. It will lose instantly all the excess of force which it had 
acquired by the influence of the iron. Indeed, under this influ- 
ence the two magnetisms, partly disguised by those of the iron, 
can exist in a state of decomposition which the coercive force 
alone is no longer able to maintain ; the magnet, therefore, aban- 
doned to itself, must return to the maximum of magnetic force 
which the nature of its substance admits, that is, to its state of 
saturation, and what is very important to remark, the restitution 
appears to take place instantaneously. 

This principle has been very advantageously employed to in- 
crease the force of natural and artificial magnets, by fitting them 
up with what is called armatures. An armature consists of pieces 
of very soft iron, applied to the polar faces of the magnet, 
which, becoming themselves magnetic by influence, increase its 
energy every day. Let us take a loadstone of a square form, 
Fig. 102. suc h as AA'"BB'", having AA'" for its north, and BB"' for its 
south pole. Let us now suppose at first, that we apply to the 
first of these poles an armature of soft iron A' A" A'", of the form 
indicated by the figure, the natural magnetisms of this plate will 
soon be decomposed ; its boreal magnetism will be attracted by 
the austral magnetism which prevails in AA'", and its austral mag- 
netism will be repelled, so that this last will predominate over all 
the exterior surface A' A" A'" of the plate of iron, but principally 
in the most distant extremity A' A", which is called the foot of 
the armature. Let us now envelope with a similar armature the 
other pole of the magnet BB'" ; a similar decomposition will be 
produced, and the foot B'B" will acquire boreal magnetism. 
After some time, the influence of the armature will have produc- 
ed a perceptible decomposition of magnetism in the particles of 
the magnet which it envelopes, and this will be considerably 
stronger. This envelope should not be too thin ; for, all circum- 
stances remaining the same, the developement of magnetism 
capable of being produced in a piece of iron depends upon its 
mass ; moreover it should not be too thick, as the greatest ener- 
gy of the action does not reside upon the lateral surface, but in 
the feet A' A", B'B". The advantages of this circumstance we 
shall soon have occasion to explain. Generally, the proper 
thickness of the armature of each magnet is to be determined by 
experiment. It is very evident that this armature ought to be 



Different Methods of Magnetizing. 233 

made of soft iron, in order to facilitate the decomposition of mag- 
netism. Steel and hard iron would be^ltogether hurtful, though 
they have been recommended by some writers. 

195. The arming of magnets not only increases their force by 
a new disengagement of the magnetism which it excites, but it 
increases it also by giving a better direction to the magnetic forces. 
Let us suppose, for example, that we wish to make use of the 
unarmed magnet A ABB in magnetizing the bar a 6, by present- Fig. 103. 
ing the northern face BB to one of its extremities. It is manifest, 
from an inspection of the figure, that the greater number of 
points of this face, as well as the points beyond them, will act 
very obliquely upon the bar, and will consequently have little 
influence in decomposing, by their attraction, the magnetism of 
its particles in the direction of its length a b. Besides, the au- 
stral surface AA, which is parallel to the first, will oppose this 507. 
effect by its contrary influence ; and though its action is more 
feeble, because it is exerted at a greater distance, yet it has the 
advantage of a more favourable direction, from its acting at 
smaller angles with the length of the bar. On the contrary, 
when the opposite energy of the two poles is turned aside, and 
carried in a great measure into the feet of the armature, let the 
bar a b be held in the prolongation of one of the feet B'B", as 
represented in figure 102 ; we shall first perceive, that the action 
of this pole, concentrated as it is in the foot, will act much more 
nearly in the direction of the length of the bar, than the large 
surface of the magnet BB did ; and, on the contrary, the action 
of the other pole AA\ carried into the corresponding foot A'A'\ 
will act much more obliquely on the bar than it did in the case 
of parallelism ; the latter will, therefore, have much less influ- 
ence in opposing the immediate action of the foot to which the 
bar is applied. By these means, we are able to communicate 
to a bar a much higher degree of magnetism than could have 
been done with the same magnet unarmed. We shall be con- 
vinced of this if we compare by the method of horizontal oscilla- 
tions the intensities of the directive forces which the same bars 
acquire, when magnetized successively in these two ways. In 
order to preserve a magnet of this kind, we must apply to its 
two poles a parallelopiped of soft iron, which answers the pur- 
pose of an armature, and which is taken away when we mean to 
employ the power of one of its poles. The preserving influence 

E. fy M. 30 



234 Magnetism* 

of this parallelopiped is founded upon the same principle as the 
use of the armature. 

In this manner we obtain the highest degree of magnetism 
•which can be produced by simple contact ; but the necessity of 
communicating to compass needles the highest possible energy 
has given rise to various other methods, which we shall proceed 
to describe. 

196. The first method of making artificial magnets, which was 
for a long time almost the only one, consists in applying the plate 
or bar of steel at right angles to one of the poles of either a 
natural or artificial magnet, and rubbing it upon this pole in the 
direction of the length of the bar, as represented in figure 105. 
In order to estimate the effect of this method, let us consider 
the bar a 6, when its extremity 6 is first applied to the pole 
A, of the magnet AB, and let us suppose that A is the south 
pole of this magnet. In this case, the austral action of the part 
CA, predominating over the boreal action of the portion CB, 
will produce in b a decomposition of the natural magnetisms of 
the plate ; the austral magnetism of each particle will be repelled 
towards a ; the boreal magnetism will be attracted towards 6, 
and it will form in b a north pole. But when the pole A quits 
the extremity 6, and begins to move over the succeeding points of 
a b, it will produce upon each of them the very same effect ; that 
is, it will attract the boreal magnetism of each particle to the 
actual point of contact, and will repel from it the austral magnet- 
ism. But the continuance of this repulsion will at last be found 
to have destroyed entirely the first decomposition of magnetism 
which had been produced by immediate contact with the ex- 
tremity 6. According as it advances on the plate, the pole will 
continue to produce the same effect, and to destroy successively, 
by its influence at a distance, the decomposition which had been 
produced by contact with the points previously touched. But 
this cause of destruction will not take place for the extremity a 
of the bar which arrives last at the pole A, in the position 6' a'. 
The effect of immediate contact will follow in its stead; and 
upon quitting the magnet, it will preserve the developement of 
boreal magnetism which had previously taken place. Hence it 
is almost entirely to this last effect that the action of the present 
method is limited ; and consequently, we should not expect more 
success than from the method of simple contact which we first 



Different Methods of Magnetizing. 235 

employed. This result may be confirmed by experiment ; for 
if we measure by means of the torsion balance the directive 
forces obtained by this method, and compare them with those 
derived from the other methods which we are about to describe, 
we shall find that it is not capable of magnetizing to saturation 
any needles but such as are very thin. 

This method has also the disadvantage of producing consecutive 
points frequently and easily, like the method of simple contact, 
particularly if the plate of steel is long and hard, and the mag- 
net is kept longer on one point than on another. This last cir- 
cumstance is sufficient of itself to produce these points ; for if we 
take a plate of steel that has been regularly magnetized, that is, 
which has in each of its halves the same kind of magnetism, and 
apply one of the poles of a needle to any part of its length, 
a pole will be created in these points of an opposite name to that 
of the pole applied ; at least, if the magnet is more powerful than 
the plate. If the magnet is very powerful, and the plate not 
thick, it is sufficient to apply it to the middle of its length, in 
order to create in this point a pole, and two opposite poles at the 
two extremities, as may be verified by the trial needle a /3, which 
we have already employed. 

197. The method of making artificial magnets which we have 
described presents a remarkable phenomenon. When the plate 
a fe, has been thus rubbed upon one of the poles of a very strong 
magnet, on the north pole, for example, and has consequently 
received a high degree of magnetism, let it be rubbed along its 
whole length, and in the same direction, upon the homologous 
pole of a weak magnet. We should be led to believe that this 
operation, performed in the same direction as the first, would 
augment its magnetic state, or at any rate, would leave it as it 
was before ; but it actually diminishes it, and the magnetism is 
reduced to the same intensity as if the plate had been magnet- 
ized by the weak magnet. In order to understand this phenom- 
enon, we must consider, that the second magnet in touching suc- 
cessively every point of the first half of the plate, creates for a 
moment by its contact, a magnetism opposite to that which the 
first magnet had left in it ; at least, if we suppose that the sec- 
ond magnet is formed of steel sufficiently hard to prevent its 
own magnetism from being destroyed by that of the plate. This 
local inversion is produced constantly, though the second magnet 



236 Magnetism, 

is weaker than the first, because it acts successively on each 
point by immediate contact, whereas the first magnet produced 
the final state of magnetism by acting at a distance. While, 
therefore, the second magnet rubs upon the first half of the 
plate, by touching it with its north pole, each point which it 
touches is, at first, brought back to its natural state, and then passes 
to the austral state, and receives afterwards, by influence at a 
distance the final degree of boreal magnetism which the magnet is 
capable of giving it, by combining its action with that magnetism 
of which it had already been rendered free. But, as these suc- 
cessive changes of free magnetism cannot take place in the one 
half, without creating corresponding changes in the other, it is 
obvious that the plate, after having experienced this disturbing 
force over the whole of its length, will be brought back precisely 
to the same degree of magnetism as if it had been touched only 
by the second magnet. It is manifest, also, that this reduction 
will not take place if the north pole of the second magnet touches 
the plate only on its south half; for then the magnetism of the 
latter will be rather augmented than diminished. For the same 
reason, there will be no longer any diminution, if 4he magnetism 
of the plate were sufficiently strong to destroy that of the second 
magnet and reverse its poles. The extreme case of this suppo- 
sition will happen when this magnet is made of very soft iron, 
in which the decomposition and recomposition of the natural 
magnetism may take place with extreme facility ; for then, as 
this iron passes over the different points of the plate, it will ac- 
quire, at the moment of contact, a magnetism opposite to that of 
the point which touches it, and consequently, by its reaction, it 
will tend to augment the species of magnetism which this point 
already possessed. This would be, as it were, a moveable ar- 
mature, applied in turn to different points of the plate ; and we 
think there can be no doubt that this repeated friction, instead 
of diminishing the magnetism of the plate, would soon bring it to 
its maximum. 

198. After many fruitless attempts to modify and bring to perfec- 
tion the method of making artificial magnets by simple contact, 
the first step towards more complicated and better methods, was 
made in 1745 by Dr Gowan Knight, of London. Having joined 
by their ends two bars strongly magnetized, the north pole of 
the one touching the south pole of the other, he placed upon 



Different Methods of Magnetizing, 237 

these bars, and in the direction of their length, a small bar of 
steel, tempered at a cherry-red heat, the middle of which cor- 
responded to the point of junction of the two large bars ; and, sep- 
arating the bars, he rubbed each of them upon the correspond- 
ing extremity of the small bar, which was found to acquire by 
this operation a more intense degree of magnetism than had 
hitherto been obtained. 

In this process, each magnet acts upon the half of the small 
bar which it passes over, as in the first method ; but, in that 
case, the influence of the same magnet acted alone over all the 
length of the plate, in order to develope the two magnetisms; 
whereas, in the new method, this decomposition is favoured by 
the presence of the other magnet ; for in all the points which lie 
between them, their influence is combined, and the kind of mag- 
netism which is attracted by the one towards one extremity of 
the bar, is at the same time repelled by the other towards the 
same extremity. By employing this method, and making use of 
large bars strongly magnetized, it is found that small bars when 
they are short, and not very thick, acquire nearly a maximum 
of magnetism ; but it is impossible by this process to magnetize 
a long bar to saturation. 

The discovery of Dr Knight led, at this period, several philoso- 
phers to seek other means of obtaining the same degree of mag- 
netism in larger bars. M. Du Hamel, of the Academy of Scien- 
ces of Paris, having united himself with Antheaume in this inquiry, 
contrived the following method. Having placed parallel to each 
other two bars of steel of the same length, AB, A'B', he united Fig. 106. 
their extremities by small parallelopipeds F, F', of very soft iron, 
so as to form a right-angled parallelogram. He then took two 
bundles of bars a 6, a' b', previously magnetized, and united their 
poles of different names towards the middle of one of the bars of 
steel ; after which, inclining the bundles as represented in the 
figure, he carried them towards each extremity of the bar ; 
and by a successive repetition of this friction upon each bar of 
steel AB, A'B', he obtained a considerable degree of magnetism. 
In this arrangement, each bundle acts upon the half of the bar, 
which it passes over as in the first method. The employment of 
two bundles instead of one has also the same advantage as the 
method of Dr Knight; but the application of two small bars of 
iron to the extremities of the bars of steel is a very important 



238 Magnetism. 

addition ; for as soon as the bars of steel have acquired any 
degree of magnetism, these small bars of soft iron, magnetized 
also by influence, will themselves act upon the steel bars like a 
real armature ; they will fix in each of their extremities the 
magnetism already developed ; and, neutralizing it, they will 
give to the moving bundles a greater degree of facility in effect- 
ing a new decomposition of the magnetism by a new friction. 
There was now only one step to be taken, in order to give to 
this method all the perfection of which it is susceptible. It 
was only necessary to substitute in the place of the small 
bars of soft iron two strong magnets, with their poles opposite 
to each other, in order to retain and neutralize still more 
strongly the magnetism previously decomposed by the rubbing 
bundles. This improvement, as we shall presently see, was 
made by JEpinus. But when large magnets cannot be obtained, 
the method of Du Hamel is the best which can be employed for 
magnetizing compass needles, and plates which are not more 
than one-f ighth of an inch thick, provided that the moving bun- 
dles are strongly magnetized. 

199. About the same time that M. Du Hamel was occupied 
wilh these researches at Paris, Mr Michel and Mr Canton were 
pursuing the same object in England. 

Mr Michel employed two bundles of bars, strongly magnet- 
ized, and placed parallel to each other, the poles of different 
names being united at each extremity ; in such a manner, howev- 
er, that there remained an interval between them of about one- 
third of an inch. He then placed, in the same straight line, sev- 
eral equal bars which he wished to magnetize, and caused 
to pass over these bars at right angles, and in the direction of 
the line formed by them, one of the extremities of the double 
bundle. By this method, he found that the intermediate bars 
in the chain acquired a great magnetic force. The magnet- 
ism, however, which is thus obtained, never rises to the max- 
imum of saturation. 

The different bars placed in contact by their extremities, have 
here the same effect as the small bars of iron employed by Du 
Hamel. They perform the part of a real armature ; but as the 
nature of their substance does not permit the free developement 
of magnetism, they do not become magnetic, and they do not act 
till they have been touched by the moving bundles. Hence we 



Different Methods of Magnetizing*, 239 

perceive why the intermediate bars in the series are the only 
ones that are strongly magnetized, for they are the only ones 
which are armed. In this respect the method of Michel returns 
to that of Du Hamel, and is perhaps interior to it ; but it pre- 
sents another modification which deserves to be examined, viz. 
the employment of two parallel bundles kept at a constant dis- 
tance by their opposite poles, and rubbing simultaneously over 
the whole extent of the bars. In order to conceive distinctly the 
effect of this arrangement, let us represent the two bundles by 
AB, B'A'\ let us suppose that the poles pass over the bar of Fig- 107. 
steel B"A", and let us analyse their action upon the points of this 
bar, both within and without the interval which they comprehend. 

We shall first consider the bundle AB, which we shall suppose 
not to have any consecutive points, so that the half CB, which 
is the most distant from the bar shall possess the boreal magnet- 
ism, and the nearest half CA the austral magnetism. If m is 
any particle of the bar A"B", all the points of the bundle AB, 
whether this particle be within the bundles, as in figure 107, or 
without them, as in figure 108, will exert upon the natural magnet- 
isms of this particle a boreal or austral action, and will tend to 
separate them according to the nature of the action. But if the 
two halves of this bundle possess nearly equal degrees of mag- 
netism, as they must do, since we suppose that the point of indif- 
ference falls nearly in the middle of its length, it is evident that 
the austral action must predominate over the other, because the 
points which exert it are nearer to the particle m, so that the 
final and total action of the bundle AB, will have for its result- 
ant an austral force, directed according to a certain line o m, 
which will cut AB in the austral portion, at a little distance 
from its extremity ; for, in magnets that have no consecutive 
points, the quantity of free magnetism is the greatest possible at 
the extremities themselves, and thence decreases towards the 
centre with extreme rapidity, like the free electricity in the 
tourmaline, and in insulated electrical piles. 93 * 

Now, if we consider the action of the other bundle A'B' upon 
the same particle, we shall see, in like manner, that there will 
result from it a single boreal force, whose direction, represented 
by m </, will cut this bundle in its northern half at a little dis- 
tance from its extremity. 



240 Magnetism, 

In order to find the joint effect of these two forces in the direc- 
tion of the bar's length A"B", we must decompose them in that 
direction. If we represent them by mr, ra?- 7 , each of them will 
give a force m/, mf, perpendicular to the direction of the bar, 
and a boreal or an austral force ms, mn, in the direction of its 
length. These last forces are the only ones with which we are 
concerned, as they alone determine the longitudinal decomposi- 
tion of the magnetism. If we compare the figures we shall 
find that if the particle m is situated within the bundles, as in 
figure 107, the two forces ran, m s, unite to decompose its natu- 
ral magnetisms in the same direction a b ; the boreal magnetism 
being attracted in the direction of the extremity b of the particle, 
situated towards the part B" of the bar, and the austral magnet- 
ism in the direction of the extremity a, situated towards the part 
A" of the bar. It is likewise evident, that this effect will take 
place upon all the other parts of the bar to which the two bun- 
•dles may be carried. If the particle m, on the contrary, is situated 
without the interval comprehended between the two bundles, as 
in figure 108, the longitudinal actions of the bundles will oppose 
each other, and the action of the nearest one, for example, will 
predominate in consequence of its proximity, and there will re- 
sult a momentary decomposition of magnetism opposite to that 
above supposed ; for the austral magnetism will be carried in 
the direction of the extremity b of the particle, situated towards 
the part A" of the bar, and the boreal in the direction of the 
extremity a situated towards the part B". This decomposition, 
however, produced by the difference of the forces, will always 
be weaker than the first, which is produced by their sum ; and 
this will be particularly the case, if the rubbing bundles are 
placed at a small distance from each other ; for their opposite 
influence will then become almost equal upon the points of the 
bar which are ever so little distant from their poles A, B'. The 
Fig. 108. feeble developement of magnetism which thus takes place in ra, 
cannot resist the combined action of the two bundles when they 
Fig. 107. are carr ied to ra, and that point is comprehended between 
Fig. 107. them ; and reciprocally, when they leave the point ra, they can- 
not destroy in it all the developement of magnetism which they 
had before produced by exerting upon it their united influ- 
ence. When this operation is frequently repeated from one end 
of the bar to the other, it will always tend to excite an in- 



Different Methods of Magnetizing. 241 

creasing developement of magnetism; and experience proves 
that this developement becomes very considerable. In order 
that it may be equal in the two halves of the bar, the united 
bundles must be first applied at iis centre, and an equal number 
of similar applications must be made upon each half of the bar. 
The bundles being then brought back to the centre, they must 
be lifted up vertically, so as not to disturb the longitudinal effect 
which had been previously produced. This method, called by its 
inventor the method of double touch, has obtained a great de- 
gree of celebrity. 

200. Mr Canton published a modification of this method ; but 
it had only the appearance of novelty. He formed at first, as 
Du Hamel did, a right-angled parallelogram, by uniting the ex- 
tremities of the two steel bars with pieces of soft iron ; he then 
touched these bars with two parallel bundles, united according 
to the method of Michel, and then separating the bundles, and 
inclining them on both sides to the bar, he moved them each way 
towards the extremities* But from what we have already said of 
the effect of repeated frictions with magnets of unequal strength, 
it is obvious that the last operation, with the inclined bundles, 
is the only one which determines the final magnetic state of the 
bar. The preceding modification, therefore, of the method of 
double touch is quite useless, and the operation, deprived of this 
superfluous addition, is identically the same with that of Du 
Hamel. 

201. iEpinus made a modification of the method of double touch 
much more chappy, and better contrived. He caused the poles 
of the two bundles to move at a small distance from each other 
without ever separating them ; but he inclined the bundles in 
opposite directions, as Du Hamel had done, and as is represent- 
ed in figure 109. By this means, the resultant of their action 
upon each particle m became more oblique to the surface of the 
bar, and consequently the part of this resultant which is ex- 
erted in a longitudinal direction became more considerable. It 
is true, indeed, that the proper action of each point of the 
bundle was at the same time diminished, because, it being neces- 
sary, in order to incline it, to turn it upon one of its edges, 
this motion necessarily separates each of its points from the 
particle m upon which it was to act. But notwithstanding this 
circumstance, we find that to a certain limit of inclination, the 

E. & M. 31 



242 Magnetism. 

oblique position is on the whole advantageous. Experiment 
alone is capable of indicating the most favourable limit, ^pinus 
decided upon an inclination of 15 or 20 degrees to the surface 
of the bar, and this seems to be the most advantageous, though 
from the nature of a maximum, any small variation in the angle 
will not perceptibly alter the result. iEpinus added to this mod- 
ification the employment of armatures, but he advantageously 
substituted, in place of the soft iron of Du Qamel, two strong 
magnets, with their opposite poles united, as we have already 
stated. The combination of these two operations constitutes 
the method to which his name has been given. In examining 
the results which it produces, it has been found superior to every 
other method, when we wish to magnetize very large bars with 
bundles of plates that have a feeble magnetism ; but it is neces- 
sarily attended with some inconveniences, which it is of import- 
ance to notice. The first of these is, that it never produces a 
developement of magnetism perfectly equal in the two halves of 
the bars to which it is applied. If we place these magnetized 
bars indeed horizontally, under a sheet of paper covered with 
very fine iron filings, we shall see from the manner in which 
they are grouped, that the neutral point is not exactly in the 
middle of the bar, but is, as Coulomb observed, somewhat re- 
moved toward the extremity last magnetized. 

It appears, in the second place, that the method of iEpinus 
produces consecutive points, in very long plates, more readily than 
that of Du Hamel. These alternations have, indeed, in all cases 
very little energy ; but they nevertheless diminish the directive 
force, which is a matter of great inconvenience in the construc- 
tion of compass needles. The same thing may be said of the 
other small inequality in the distribution of the magnetism ; and, 
therefore, it is much better to magnetize needles by the method 
of Du Hamel, which is completely exempt from these two faults, 
and reserve the method of JEpinus for large bars, to which we 
wish to communicate a very great force, for then it is of little 
consequence whether or not the neutral point is placed exactly 
in the middle of the bar. 

202. By thus taking from each of these methods what is most 
useful, and adding to them the information obtained from long 
experience, Coulomb arrived at the following arrangements. 

In order to form fixed bundles, he employed for each ten bars 



Different Methods of Magnetizing. 243 

of steel tempered cherry-red, having a length of about 21 or 22 
inches, a breadth of about six-tenths of an inch, and a thickness 
of one-fifth of an inch. He magnetized them as highly as he could 
with a natural or artificial magnet, and then, uniting them by 
their poles of the same name, he formed two beds of five bars 
each, separated by small rectangular parallelopipeds of very soft 
iron, which performed the part of a common armature, and 
which projected a little beyond their extremities. See figure 1 10. 

I have found that we might advantageously substitute for these 
paralellopipeds, plates of soft iron, which unite at the extremity 
of the magnet, into one mass, forming a truncated pyramid. This 
disposition of the bundles, by which the magnetic forces are 
concentrated, is represented in figure 111. 

The moving bundles he commonly formed of four bars tem- 
pered cherry-red, about 16 inches long, and one-fifth of an inch 
thick, and six-tenths of an inch wide. After magnetizing them 
as strongly as possible, he united two of them by their widths, 
and two of them by their ihicknesses, which gave to each bundle 
a width of one inch and two-tenths, and a thickness of two-fifths. 
It is evidently advantageous to apply to them a common arma- 
ture of soft iron of the same form as that of the fixed bundles. 

Both the fixed and moveable bundles were made of a steel, 
well known in commerce from a stamp of seven stars. It is of 
a moderate quality, but Coulomb observed, as had already been 
done before, that every kind of steel, provided it is not of a very 
bad quality, takes nearly the same degree of magnetism. We 
shall only remark, that as bars are always bent a little in tem- 
pering, they should be tempered at first as hard as possible, and 
then annealed to the first shade of yellow. This annealing gives 
them malleability sufficient for forming them again into shape, 
and at the same time leaves a coercive force sufficient for pre- 
serving a very energetic developement of magnetism. 

In order to magnetize a needle or a bar of any kind by means 
of this method, we begin by placing the large bundles in the 
same straight line, so that their north and south poles are turned 
toward each other, and kept at a distance equal to the length of 
this bar, as in figure 112. Each of its ends is then placed upon 
one extremity of the armature, in such manner as to lap a 
little over it ; after that, two moveable bundles are placed upon 
the centre of the bar, and inclined each way in opposite 



244 Magnetism. 

directions, so as to form with it an angle of about 20° or 30°* 
Then, if we wish to employ the method of M. Du Hamel, we 
must cause each bundle to move over the half of the bar on 
which it is placed ; but. if we wish to employ that of iEpinus. we 
do not separate them, but place them, together with a small piece 
of wood or copper between them, in order to keep their oppo- 
site poles at a distance of about one-fifth of an inch ; and holding 
them in this manner, with the same inclination as in the other 
method, they are moved successively from the centre to each 
extremity, so that the number of applications upon the two halves 
of the bar may be equal. After the last motion by which they 
are brought to the centre, they are withdrawn perpendicularly, 
and the same operation is repeated upon each of the other surfaces. 

203. If the bars which compose the bundles have not been at 
first magnetized to saturation, which will generally happen when 
we have not at our command an apparatus like the preceding, their 
assemblage will produce, in bars subjected to their action, a 
much stronger degree of magnetism than they themselves pos- 
sess. These new bars may then be used in forming other bun- 
dles, stronger than the first ; and if we have not yet attained the 
maximum of energy, we may repeat the operation a second, a 
third, or even a fourth time, till we have obtained bundles as 
strong as we can desire. 

We have said that each moving bundle was composed of four 
bars. When we wish, however, to magnetize very thick bars, 
we must unite a greater number, arranging them in steps retreat- 
ing about half an inch in the direction of the thickness, as is 
shown in figure 1 13. This arrangement is founded on the fact, 
that the greatest developement of magnetism takes place at the 
extremities of the bars. In this case, the bar nearest to the cen- 
tral one tends to maintain, and even to augment, at its extremity, 
the developement of magnetism which already resides in it. 
The third bar produces the same effect upon the second, and so 
on with the rest. In order to concentrate still more the action 
of these bars, we may unite them by pyramidal armatures of 
soft iron, resembling those we have before described. 

When we have finished the operation, either with the system 
of fixed or moveable bars, we must place the two of each pair 
parallel to each other,' similar poles being in opposite directions, 
as represented in figure 1 1 4. The poles are then joined by par- 



Different Methods of Magnetizing. 245 

allelopipeds of soft iron, which, becoming magnetic by influence, 
neutralize the magnetism of the bundle, and tend to increase 
rather than to diminish it. The effect is precisely the same as 
that which we have explained above, in speaking of the increase 
of force which natural loadstones acquire by time, when the 
feel of their armature are united by pieces of soft iron. 

Coulomb has verified all these theoretical considerations by 
the most delicate experiments, in the course of which he has 
applied the several methods of making artificial magnets to bars 
of the same nature and the same dimensions ; and the intensity 
of the magnetic charge was readily measured by means of hori- 
zontal oscillations, as heretofore explained. 

These methods show that the experiments of Du Hamel and 
iEpinus, are superior to all others ; inasmuch as they impart an 
equal degree of magnetic power with a much smaller number of 
moveable bars. It will be seen that the two methods are equally 
good, so long as we wish to operate on bars of only about an 
inch in thickness ; but in applying them to bars of greater thick- 
ness, the method of iEpinus is decidedly the best. It would be 
of little use to increase, the thickness of the bars in the magnetic 
apparatus to beyond three or four inches ; for experiments show 
that we shall obtain a much greater intensity of magnetic force 
by uniting many small bars magnetized separately before being 
united ; and this evidently results from the fact, that we can com- 
municate a much more powerful magnetic force to a single bar, 
than to a bar placed between a number of others. 

204. In the foregoing remarks I have supposed the process of 
magnetizing to take place at the ordinary temperature of the 
atmosphere. But perhaps a still greater developement of mag- 
netism might be obtained by raising the temperature of the bars 
while the process is going on, or by altering the nature of 
the substances used as bars. The first suggestion was made 
by Robison ; the second has been practically applied by 
Knight ; and it would seem, that his success has been such, that 
the experiment deserves to be submitted anew to the most rigor- 
ous examination. The process of Knight differs from the ordi- 
nary one, in requiring as a substitute for steel, a paste made of 
the deutoxide of iron pulverized, and mixed with linseed oil. This 
paste dried by a gentle heat, acquires after a few weeks, accord- 
ing to him, an intensity of magnetic force of which it is exceed- 
ingly difficult to deprive it. 



246 Magnetism, 



General Distribution of Free Magnetism in Wires Magnetized by 
the method of Double-touch — Laws of Magnetic Attraction and 
Repulsion. 

205. If, after having magnetized, by the method of Du Hamel 
or that of iEpinus, a steel wire 15 or 20 inches long, and one or 
two lines in diameter, we examine what weight it is capable of 
supporting at different points of its length, we shall find that this 
weight goes on increasing from the extremity of the wire for the 
space of four or five lines, beyond which it diminishes rapidly, so 
as to become almost insensible at the distance of two or three 
inches from the extremity. These weights will also be found to 
be equal towards the ends of the wire ; and hence it follows, as we 
had foreseen, that the most intense quantities of free magnetism 
are distributed towards the two extremities, and at a small dis- 
tance from them, and that they are sensibly equal there, — a dis- 
tribution perfectly analogous to that of free electricity in the 
tourmaline and electric piles. 

This important result may be proved in the most satisfactory 
manner by the torsion balance. The experiment, as perform- 
ed by Coulomb, is represented in figure 115. Having adapted 
to the stirrup of the magnetic balance a suspension wire, whose 
force of torsion is very small, we place in it a steel wire a 6, 
strongly magnetized by the method of Du Hamel or that of Myi- 
nus. In the direction of the magnetic meridian of this wire, which 
ought to correspond to the zero of torsion, a vertical rule RR of 
wood or copper, one or two lines thick, is so fixed that the ex- 
tremity a of the horizontal wire may come close to it, when it is 
brought back to the magnetic meridian. On the other side of 
this rule, and alpng a grove made in it for the purpose, a mag- 
netized steel wire a' &', such as we have above described, is made 
to pass vertically, so as just to present its homologous pole a' to 
that of the needle. The needle will at first be repelled by the 
similar magnetism of a', but it is forcibly brought back to the rule, 
by twisting the suspending wire in such a manner that there shall 
remain only the thickness of the rule, or a distance of about 
two lines between the nearest points of the wires. But since the 
wire a' b', which we have placed behind the rule, is vertical, 



General Distribution of Free Magnetism by Double Touch. 247 

while the wire a b is horizontal, the several points on each side, 
which are distant four or five lines from the intersection, contri- 
bute very little to the repulsion, on account of the distance, 
and the obliquity with which they act ; so that the force of tor- 
sion which is required in order to maintain the contact, must 
depend principally on the quantities of free magnetism which 
exist in the two needles, from the point of intersection to a 
distance of two or three lines on each side of this point. By 
thus making the wire a' h' pass vertically along the rule, present- 
ing successively its several points at the small distance of two 
lines from the same point of the wire a 6, whose action remains 
constant, the force of torsion which it is necessary to employ in 
order to preserve the position of a 6 against the rule, will be, in 
each case, a very exact measure of the intensity of free magnet- 
ism in the point of the wire a! b' which corresponds to the inter- 
section. In making this experiment it will be found, that if eight 
circles of torsion are necessary when the intersection is two lines 
from the extremity of the wire a' 6', two or three circles only 
will be necessary at two inches; and when the extremi f y of the 
wire a' b' is three inches above or below the horizontal plane of 
a 6, the repulsion is almost nothing. It follows, therefore, from 
this trial, that the free magnetism of a' b' is chiefly concentrated 
upon the three first inches from the extremity. A similar re- 
sult will be obtained from the attraction of the opposite poles; 
and if the vertical wire has been regularly magnetized by the 
process of double touch, it will be found that the attraction of 
the pole b is sensibly equal to the repulsion of the pole a' ; but 
it is necessary to observe, that in order to obtain correct results, 
we must employ only ijeedles or wires of excellent steel strongly 
tempered ; and we must take care not to give them a high de- 
gree of magnetism ; for without these precautions, the points of 
intersection being only two lines distant, the reciprocal influence 
of the needle and the steel wire may develope in these points 
new quantities of magnetism, so that the intensities of their attrac- 
tion and repulsion would not remain constant during the experi- 
ment. 

If, in the preceding experiment, we employ two similar wires, 
24 inches long, and placed so that the points of intersection shall 
be 10 or 12 lines from the extremity ; then, by bringing together 
their homologous poles, there will be a repulsion. But this repul- 



248 Magnet 



ism. 



sion will arise almost entirely from the two or three inches of 
length upon which the magnetism is most developed ; and the effect 
will be produced almost entirely by the contiguous poles ; for 
the action of the two others will be extremely weak, both on 
account of the length of the two wires, and on account of the 
obliquity of their direction, which will be considerable if the two 
contiguous poles depart only a small distance from each other. 
These poles are therefore placed in the most favourable manner 
for determining the law of their repulsion at different distances ; 
for, as they cross each other in the points where the repulsion 
is the strongest, the other portions of free magnetism which are 
situated near these points will have almost the same effect upon 
the repulsion as if they were all concentrated at the point of 
intersection, so that we shall have nearly the reciprocal action 
of two points, each of which is charged with a constant and given 
quantity of magnetism of the same kind. 

206. When, in the preceding experiment, the moveable needle 
is separated from the fixed one, it will be drawn towards it, not 
only by the torsion, but also by the attraction of the terrestrial 
magnet, which tends to bring it back to the magnetic meridian. 
We must, therefore, begin by measuring separately this directive 
force for different distances, and afterwards add it to the observed 
torsion, in order to have the total effect of the repulsion of the 
two wires. The following are the particulars of an experiment, 
as made by Coulomb for this purpose. 

Having taken two wires 24 inches long, and 1| line in diame- 
ter, he first put the horizontal one in its place, and by the 
method we have mentioned, determined the force with which 
the terrestrial magnet drew it back to |fae magnetic meridian. 
For this purpose, he turned the graduated circle twice round. 
The needle moved 20°, and therefore the torsion was 720° — 20°, 
or 700°, We have before seen that when the same needle is 
deflected by small quantities from the magnetic meridian, its 
divergencies are proportional to the forces of torsion, exerted 
upon it. Making use of this result, we conclude that in or- 
der to deflect the horizontal wire one degree from the mag- 
netic meridian, under the circumstances in which the pre- 
ceding experiment was made, it is necessary to employ a force 

700° 
of torsion equal to — — - or 35°. Coulomb now placed vertically 



Different Methods of Magnetizing. 249 

in this meridian another magnetic wire of the same dimensions 
with the first, so that if the two wires had been capable of com: 
ing in contact, they would have met at the distance of an inch 
from their extremities ; but as their homologous poles were op- 
posed to each other, the horizontal wire was repelled from the 
direction of its meridian, till the force of repulsion of the oppo- 
site poles was balanced by the combined forces of torsion and 
terrestrial magnetism, which tended to bring the horizontal wire 
to its point of rest. The following were the results of different 
trials ; 

Number of turns given to the suspending wire, Observed angles of 

by means of the graduated circle. repulsion. 

24° 

3 17° 

8 12° 

The first experiment expresses the angle through which the 
moveable wire was immediately driven, reckoning from the 
zero of torsion. When it stopped in this position, it was urged 
towards the zero by a force of torsion of 24°, plus the directive 
force of the terrestrial magnet for 24°, namely, 24 X 35°, or 
840°. The total repulsive force was therefore 864°. 

In the second experiment, the graduated circle was turned 
three times in a direction contrary to the 24° first produced ; 
but in spite of this great torsion, the moveable wire, repelled by 
the fixed one, was deflected 17° from its magnetic meridian ; so 
that the force of torsion was then 3 circles -f- 17° or 1097°, and 
adding to ti.is the directive force for 17°, which is 17 X 35°, or 
595°, we obtain for the total repulsive force 1097° + 595°, or 
1692°. 

In the third experiment, the wire was twisted through eight 
circles. The magnetic wire stopped at 12° from its magnetic 
meridian ; and therefore the torsion was eight circles + 12°, or 
2892°, to which adding the directive force, or 12 X 35° = 420°, 
we have for the total repulsion 3312°. 

In these experiments, therefore, when the arcs of repulsion are 
sufficiently small, so that they may be reckoned equal to their 
chords, the distances were 12, 17, 24; and the corresponding 
repulsive forces, measured in degrees of torsion, 3312°, 1692°, 
864°. 

E. <fr .M 32 



250 Magnetism, 

From these results it appears that the repulsive force dimin- 
ishes as the distance increases, and that it diminishes more 
rapidly than in the ratio of the distances simply ; for the third 
distance 24, is double of the first, and the repulsive force 865, is 
much less than half of 3312. Let us try, therefore, the inverse 
ratio of the square of the distances ; and by setting out from the 

first force 3312, they ought to be 3312 )i|f , and 3312 )—£-, or 

' J & (17) 2 (24) 2 ' 

1650, and 828, instead of 1692°, and 864, as obtained by ex- 
periment. The differences 42° and 36° correspond nearly to an 
error of one degree in the observed positions of the moveable steel 
wire, since the directive force is 35° for each degree of deviation 
from the magnetic meridian. Neglecting, therefore, this error, 
which we may consider as very small in experiments of this 
kind, we conclude that the reciprocal action of two magnetic 
wires decreases as the square of the distance increases ; and, 
consequently, that magnetisms of the same kind, by which this 
action is produced, repel each other according to this law. 

The small deviation which we have found between the ob- 
served and calculated repulsions does not perhaps arise from 
an error in the experiments, or any want of exactness in the 
law which we have deduced ; for the experiment is made, not 
upon magnetic points, but upon portions of the wire of a certain 
extent, the configuration of which has an influence upon the 
results. In the last experiment, indeed, where the two wires 
were nearest each other, the influence of the points lying near 
the intersection was more weakened by obliquity than in the 
other experiments ; or, in other words, there were at equal obli- 
quities more points which acted in the case of the greater dis- 
tance than in that of the smaller. But as we did not take this 
augmentation into account, we ought to find that the repulsive 
force, observed at the smallest distance, on being reduced in the 
ratio of the square of the distance, gives, for the larger distances 
repulsive forces a little more feeble than those which were actu- 
ally observed. 

207. The same experiment repeated with poles of an opposite 
name, shows that they attract one another in the inverse ratio of 
the square of the distance. This law of attraction and repulsion 
is the same in magnetism as in electricity. 



Intensity of Magnetism produced by Double Touch. 251 

208. It follows from these results, that Coulomb has legitimately 
omitted in his experiments the action of the distant poles ; for as 
the needles were two feet long, the greatest arc of repulsion 
which was 24 degrees, corresponded to a distance of 5 inches 
between the poles which were directed toward each other ; and 
consequently the other poles were at least four times as distant 
from those which were directed toward each other, as these poles 
were from one another. Their direct action consequently, was 
at least sixteen times weaker ; and it was even still more reduced 
by the extreme obliquity of the direction in which it was exerted. 
The case would not be the same if the experiment had been 
made with shorter wires. It would then have been necessary 
to take account of the reciprocal action of these two poles, and 
the length of lever by which each of them acted ; this would 
lead us into complicated processes which are avoided by em- 
ploying longer needles. 



Of the Intensity of Free Magnetism in each Point of a Needle Mag- 
netized to Saturation by the method of Double Touch. 

209. Having found infallible methods for developing, in bars 
of iron and steel, all the magnetism which they can acquire, and 
for preserving it in a durable manner, we shall now determine 
experimentally the magnetic state of each of these points. 

For the sake of simplicity, we shall begin with the case of a 
cylindrical steel wire AB, of a very small diameter, and regu- Fig. 116, 
larly magnetized by the method of double touch. The de- 
velopement of magnetism will then be perceptibly equal, but of 
an opposite nature in its two halves, and will decrease rapidly 
in each of them, from the extremity towards the centre. If, 
therefore, we erect at different points of the wire, perpendicular 
ordinates, to represent the intensity of free magnetism, whether 
boreal or austral, these ordinates will commence by being noth- 
ing at the centre, from which they will go on increasing equally 
and slowly on the two sides. At a certain distance they will 
increase rapidly towards the extremities of the wire, where 
they will reach their maximum. This is all our experiments 
permit us to conjecture. 



252 Magnetism. 

In order, however, to determine the value of these ordinafes*> 
Fig. 117. we must suspend by a single fibre of silk, a small trial needle a&, 
previously magnetized ; and after having allowed it to place 
itself in the magnetic meridian, we must present to it, in the 
same meridian an opposite pole of the wire AB, held vertically at 
a small distance from the pole a. This operation will not change 
the direction of the needle a b : but if we make it deviate, in the 
least degree, from its meridian, it will return to it more rapidly 
than if the wire did not act upon it, because it is drawn back 
by the combined action of the earth and the wire. The first of 
these forces may be easily measured, by making the needle 
oscillate by the sole influence of terrestrial magnetism; it will be 
proportional to the square of the number of oscillations performed 
in any given time, as a minute. If we afterwards observe, in 
the same manner, the number of oscillations, which the needle 
performs, when acted upon, both by this force, and by that of 
the wire, we shall obtain, in like manner, by squaring this num- 
ber, the total action which it experiences ; and by subtracting 
the first square, depending on the terrestrial magnetism, we shall 
have a separate measure of the action exerted by the wire. But 
in this case, the point Jf, situated opposite to the needle, will 
have the most powerful effect, both because it is nearest to the 
needle, and because it attracts it directly in the horizontal plane 
in which it oscillates ; whereas, the other points, situated above 
and below it, act at a greater distance, and with a greater obli- 
quity. The influence of these two causes, indeed, is very feeble 
for the points of the wire which are near M ; but, if th.e action 
of one of these points is stronger than that of M, that of the point 
situated on the other side, at the same distance, will be weaker 
by nearly the same quantity; for, whatever be the nature of the 
Fig. 116. curve A'CB\ which joins the different ordinates, we may always, 
when we consider a small portion of it, substitute the straight 
line which touches it. In virtue, therefore, of this substitution, 
half the sum of the equidistant actions exerted by the points 
near J\f, will differ very little from that of M ; and therefore it fol- 
lows, that in each experiment, the part of the wire whose action 
is most energetic, will exert a total force, almost exactly propor- 
tional to that of the point M, and consequently to the quantity of 
magnetism which exists in a state of freedom. This proportion- 
ality, however, must not be extended to the extremity of the 



Intensity of Magnetism produced by Double Touch. 253 

wire, nor even to its immediate vicinity ; for then the points sit- 
uated beyond the wire become so near, that their absence is 
sensibly felt ; and consequently, the action experienced by the 
trial needle cannot be the same as if the wire were continued. 
When the needle oscillates, for example, before the extremity B 
itself, the force which urges it is only one half of that which would 
have acted upon it, if there had been, in the prolongation of the 
wire, another equal wire ; and consequently, the observed forces 
will be nearly one half of those which would have been obtained, 
if the needle had been continued with the law of magnetism 
which it possesses. In order, therefore, that the results observ- 
ed in this case may be compared with those which the needle 
presents when it oscillates before the other points, where the 
wire acts upon it both from above and below, it will be necessary 
to double the number which represents the square of the oscilla- 
tions. This is what Coulomb himself did ; and we have satisfied 
ourselves, by an exact calculation, that this correction is very 
near the truth. 

210. It is necessary here to notice an objection which may 
now naturally present itself. When we determined the law of 
magnetic attraction and repulsion at different distances, we sup- 
posed that all the magnetism of the same pole acted entirely in 
the horizontal plane of the moveable needle, as if it were nearly 
concentrated in a single point ; whereas we have now said, that 
the action of the different points of this pole, which are above 
and below the plane of the needle, will be greatly weakened by 
the obliquity. The reason of this is, that, in these two cases, 
the distance of the moveable needle from the fixed wire is very 
different. In the preceding experiments the pole of the move- 
able needle was always removed to a considerable distance 
from the fixed needle, compared with the space over which 
the free magnetism was distributed. In the present case, on the 
contrary, this space is considerable, relative to the distance of 
the small needle, which is very near the wire. This modifica- 
tion renders the influence of obliquity much more considerable. 
The action of the points situated above and below the plane\>f 
the trial needle, decreases, therefore, with much more rapidity. 
The total action is always nearly the same as if the wire were 
continued indefinitely on both sides of the plane of the needle, 
and with the same magnetic intensity which resides in the point 



245 Magnetism. 

that is actually before it. Hence we see why the action thus 
observed is sensibly proportional to the quantity of free magnet- 
ism which exists in this point. 

In making these experiments, two important precautions are 
necessary. The first consists in employing wires so long, that 
in observing the action of one of their extremities upon the needle, 
there may be no occasion to take account of the action of the 
other extremity. The second precaution is, that the needle, 
though small and easily moved, may also be so strong, and 
made of steel so hard, that its magnetism shall not be perceptibly 
modified by the action of the wire ; for if this change take 
place, the experiments made before different points would not be 
comparable, since the part of the action which depends on the 
needle would vary. This actually happened to Coulomb in his 
first experiments, when he employed a small needle two lines 
long, and placed at the distance of three lines from the wire. 
This needle, abandoned to the sole action of the terrestrial mag- 
net, gave very feeble indications of magnetism ; but when it was 
placed at the distance of three lines from the wire, its magnetic 
state increased considerably, and by presenting it to each ex- 
tremity of the wire, it suddenly changed its poles. 

Warned by these phenomena, Coulomb employed a more 
powerful trial needle, which was three lines in diameter, and six 
lines in length. The diameter of the magnetic wire was two 
lines, its length 27 inches, and its weight 865 grains per foot. In 
order that it might not produce any change in the needle, he 
held it at a greater distance, namely, 8 lines, and measured the 
number of oscillations which it performed in a minute, when it 
was held before different points of the wire. Having previously 
observed the number of oscillations which it performed in the 
same time by the single action of the terrestrial magnet, the dif- 
ference between the squares of these numbers expressed, in each 
experiment, the reciprocal action of the wire and the needle; an 
action which, as we have already said, must be nearly propor- 
tional to the intensity of free magnetism in the point of the 
wire before which the needle oscillated. By placing these re- 
sults upon the corresponding abscissas, Coulomb obtained the 
curve of intensities represented in figure 118. The ascending 
form of this curve confirms all that the preceding experiments 
have led us to anticipate respecting the distribution of free mag- 



Intensity of Magnetism produced by Double Touch. 255 

netism, and the great intensity of its developement towards the 
extremities of the bar. 

211. Coulomb repeated the experiment with the same wire, 
having changed only its length, all other circumstances remain- 
ing the same. He then found, that whatever this length be, pro- 
vided it exceeds 6 or 7 inches, the three first and the three last 
inches give always nearly the same results as a wire 27 inches 
long ; so that the intensity of free magnetism is sensibly the same, 
from the extremity of these wires, to a distance of three inches ; 
after which it becomes equally weak and insensible in all of 
them ; or in other words, the curve of intensity is merely trans- 
ferred to the extremities of the wire, without changing its form in 
this part, and it is only after having descended near the axis, that 
its ordinates begin to remain nearly constant for a greater or less 
space so as to become nothing at the centre. This constancy in the 
extreme ordinates for all wires of the same kind and of the same 
size, indicates clearly that the free magnetism received in this part 
a degree of developement which it could not exceed, a result per- 
fectly conformable to the idea which we have given of the state 
of saturation. Coulomb found less constancy in the small ordi- 
nates of the curve near the middle of the wire, and he even 
ascertained, that in very long wires these ordinates varied acci- 
dentally, sometimes passing from positive to negative ; a result 
easily understood, if we consider that all these inversions consti- 
tute so many possible states of equilibrium, and that the slightest 
circumstance, such as a contact more or less prolonged during 
the process of magnetizing the wire, or even the action of the 
poles of the wire itself upon its centre, is sufficient to develope 
them. 

In considering the curve of intensity, as traced by Coulomb? 
it is easy to see that it results from the combination of two 
curves, called by geometers logarithmic, which, setting out 
from each extremity of the magnet, have their ordinates equal, 
and in opposite directions, as shown in figure 119. The varia- 
tions, indeed, of the intensities calculated in this manner for dif- 
ferent distances from the centre of the wire, is found perfectly 
conformable to observation. This law, considered analytically, 
indicates a distribution of free magnetism exactly similar to that 
of the two electricities in insulated electrical piles, when the ab- 
sorbing action of the air has equalized the tensions of their poles : 



256 Magnetism, 

and this is indeed what we ought to expect, from the perfect 
analogy which we have remarked from the beginning between 
magnets and poles of this kind. The formulas deduced from 
this approximation enable us to trace the variations of the mag- 
netic charge in wires of the same magnitude, but of unequal 
lengths, and in wires of the same length but of unequal magni- 
tude ; and, in short, in wires of any magnitude and length what- 
ever, by supposing them always magnetized by the method of 
double touch. The conclusion derived from a comparison of the 
calculated and observed results cannot, however, be explained 
here; but the reader will find it in my Traite de Physique, 

212. The experiments of Coulomb, upon which these calcula- 
tions are founded, present an equal and opposite distribution of 
magnetism in the two halves of the needle ; a distribution which 
is indeed the most advantageous for obtaining a considerable di- 
rective force, and which, therefore, we should endeavour as much 
as possible to effect. Experience, however, informs us that this 
is impossible in tempered needles, when their length is very 
great, compared with the diameter of their transverse section. 

In this case, whatever method of magnetizing is employed, 
several centres are formed, the developement of which is pro- 
bably owing to the reaction of the poles upon the points near 
the centre. In this case, the curve of intensity is no longer 
situated for the two halves of the needle on different sides of 
theaxis. It necessarily undulates above and below, as repre- 
sented in figure 120 ; and, consequently, its form can no longer 
be represented by the same analytical expression as before. 
Fortunately, there is every reason to believe that this limitation 
is not to be regretted. For, in the first place, it does not hap- 
pen in annealed needles, unless, perhaps, they very much ex- 
ceed in length those which are ordinarily used ; and with 
respect to tempered needles, if we are not constrained by some 
urgent motive to make them extremely light, there will always 
be an advantage in giving them a sufficient thickness, in order 
that the free magnetism may be of the same nature in each of 
their halves ; for, with an equality of coercive force, the deve- 
lopement of new centres always weakens the statical moment 
of the directive force for each half of the needle, and renders 
the action less energetic at equal distances from the poles. 



Best Form of Compass Needles. Ihl 

It is obvious, in general, that the distribution of magnetism in 
a needle, and the absolute degree of saturation of which it is sus- 
ceptible, depend not only on its dimensions, but also on the 
higher or lower temper which it has received. Coulomb had 
studied the influence of this last circumstance. He shows that 
we must always begin by tempering the needle at a white heat, 
whatever be its dimensions, and then, if its length is less than 
thirty times its thickness, we must leave it at this temper ; but if 
it exceeds this proportion, we must bring it back again, by an- 
nealing it to the state of dark red, in order to avoid a multiplica- 
tion of centres which its great length might occasion. 



Of the best Forms of Compass Needles. 

213. The results at which we have arrived in the preceding 
sections, should serve to direct us in making needles for com- 
passes. Although this application may be very easy, its im- 
portance entitles it to particular attention; and this will be the 
more readily bestowed, as here also Coulomb is our guide. 

The compasses commonly used, whether designed for land or 
sea, are formed of needles artificially magnetized, and provided 
with a cap at the centre, which rests on a pivot of some metal 
not magnetic. A little counterpoise placed on one arm of the 
needle renders it horizontal. It is necessary to change the place 
or size of this counterpoise as we change our latitude, the mo- 
ment of the vertical forces of terrestrial magnetism being differ- 
ent in different latitudes. Whatever be the form of the needle, 
it is easy to determine, on its surface, the horizontal direction of 
the magnetic resultant by the method already explained. If the 
needle move on its pivot with perfect freedom, it will naturally 
direct itself in such a manner as to cause the magnetic axis to 
correspond exactly to the magnetic meridian ; and consequently, 
when once known, it will exactly determine this meridian. But 
the friction of the pivot on the bottom of the cap opposes this 
tendency, and presents an obstacle which the directive force 
must surmount in order to bring the needle to the magnetic meri- 
dian ; whence it is evident, that the best construction is that in 
which the friction is least, and the directive force the greatest. 

E. & M. 33 



258 Magnetism. 

214. On the supposition that the pivots and caps are of the 
same shape, the same materials, and formed with equal care, the 
friction will depend simply on the weight of the needle ; and it 
may be measured by presenting the needle from a distance, 
while balanced on its pivot, to a magnet that draws it from the 
plane of the magnetic meridian, and observing how nearly it re" 
turns to its proper situation, when left at perfect liberty. It 
should seem that the arcs which it describes on each side of this 
plane, a great number of experiments being used, should be pro- 
portional to the force of friction. By observations of this kind 
Coulomb found that for very sharp pivots, and caps formed of 
a substance sufficiently hard, the friction is proportional to the 
power | of the pressure. 

But when by long use the pivots have become blunted, and as 
it were fitted to the excavation of the cap, which is frequently 
the case, he found the friction to be simply proportional to the 
pressure. This is the first established fact of which we are to 
avail ourselves. Let us conceive a magnetic needle of any form 
and size whatever, placed on a pivot of the above description ; 
and, without changing its length at all, let us only double its thick- 
ness, or which amounts to the same thing, cover it with another 
lamina of metal precisely similar ; the pressure on the pivot will 
be doubled, and also the friction ; but not the directive force. 
For it is manifest, and proved by experiment, that this force in- 
creases in a less ratio than the thickness, since the re-action of 
homologous poles on each other destroys a part of the free 
magnetism which each one separately possessed. The needle, 
when covered with its additional coating, will point out the mag- 
netic meridian less accurately than before ; and hence it will be 
seen that, other things being the same, the most correct needles 
are those of the least diameters. The diameter will be sufficiently 
great if it be such as to prevent the needle from being bent by 
its weight. 

215. Let us now proceed to consider the lengths of needles, 
and first, the case of those which from their dimensions and phy- 
sical state possess only one kind of free magnetism in each of 
their ends. Then the analytical law relative to the intensities, 
obtained above, shows that unless the needles are exceedingly 
short, their directive forces, the diameters being equal, are pro- 
portional to their lengths, at least if we suppose their transverse 



Best Form of Compass Needles. 259 

sections to be every where the same. But, in this case, the 
weight, and consequently the friction which results from it, are 
each proportional to the length. So that if we avoid exceedingly 
small dimensions, all needles, whatever be their lengths, have 
nearly the same degree of accuracy. This, however, is true 
only on the supposition of a symmetrical distribution of magnet- 
ism in the two arms of the needles, and a freedom from consecu- 
tive points. It is necessary then to attend to the relation of the 
length to the thickness, as well as to the state of annealing and tem- 
pering, in order that this condition may be fulfilled ; and we must 
accordingly observe the directions given in the preceding sec- 
tion. If the length of the needle be less than 30 times its thick- 
ness, we must temper it at a white heat, before we develope its 
magnetic power. If, on the other hand, its length exceed this 
proportion, we must anneal it till it becomes of a dark red col- 
our. When the length is between these two limits, it is not of 
much consequence which process we employ. The superiority 
possessed by needles having a single magnetic centre over those 
of several centres is incontestible, if we suppose the same quan- 
tity of magnetism to be developed on the whole in each case. 
But it is not impossible, that with other proportions of thickness 
and length, and other degrees of temper, the diminution of mag- 
netic force occasioned by the multiplicity of centres, may be 
compensated by the existence of a coercive force more consider- 
able than could be otherwise obtained, or by a more abundant 
developement of magnetism. It appears that Coulomb perform- 
ed a great many experiments relative to this subject, which he 
proposed to arrange in tables, so that we might know beforehand 
what were the most favourable circumstances for every variety 
of dimensions in the needles. But unfortunately, nothing has 
been found in his manuscripts sufficiently matured to be em- 
ployed in so important an undertaking ; and the subject still de- 
mands the attention of philosophers. 

216. It now remains for us to inquire, which is the most ad^ 
vantageous of all the forms of needles. Usually, they are par. 
allelograms, cylinders, or arrows. Coulomb ascertained by 
experiment, that when the weights are equal, arrow-shaped 
needles have the greatest directive force. And this might be 
naturally inferred from the reason which induced him to arrange 
his magnetic bundles by steps retreating in the direction of their 



260 Magnetism, 

thickness, as already explained. It will also be evident from the 
same principles, that there is great disadvantage arising from the 
extremities of the needles being enlarged ; and this modification, 
which some have proposed to introduce, should be steadily op- 
posed. The remarks here made are equally applicable to dip- 
ping needles. With respect to these it will also be necessary to 
employ the processes for correction by inversion, as heretofore 
made known. 



Of the Action of Magnets on other Natural Substances* 

217. We have said that iron, steel, nickel, and cobalt, were 
the only magnetic metals at present known. And indeed they 
are the only metals capable of acquiring a high and permanent 
degree of magnetism. Still if we take a small needle a third of 
an inch in length, and of about an inch in thickness, of any sub- 
stance whatever, and suspend it by a silk thread between the 
opposite poles of two powerful magnets, as represented in figure 
121, it will be found always to place itself in the direction 
of these poles ; and if we cause it to vibrate about its line 
of equilibrium, the oscillations performed in presence of the 
magnets are much more rapid than those w r hich take place in 
empty space. These little needles then are sensible to the influ- 
ence of magnetism. We shall be equally successful whether we 
employ in our experiments needles of gold, silver, glass, wood, 
or any other substance, organic or inorganic. These remark- 
able facts were discovered by Coulomb, and announced by him 
to the National Institute in May, 1812.t 

218. There seem to be only two ways of accounting for these 
phenomena. Either all substances in nature are susceptible of 
magnetism, or all possess particles of iron, or some other mag- 
netic metal from which this property is derived. But this alter- 
native is not so necessary as we should at first suppose ; for it 

rests on the assumption, that the action exhibited by the needles 

> 

t A detailed account of them is given in Biot's Traite de Physique, 
with a calculation of the forces exerted by them. 



Action of Magnets an other Substances. 261 

in question is actually magnetic ; and this cannot be positively 
affirmed. When we find that the simple contact of heteroge- 
neous bodies develope very sensible electric forces, whose very 
existence we had never before suspected, must we not regard it 
as possible, that other circumstances are capable of developing 
like or analogous forces, whose feeble effects can only be per- 
ceived by means of the most delicate instruments ; and may not 
the action observed in the little needles of Coulomb be referrible 
to some subtle force with whose nature we are yet wholly unac- 
quainted ? These questions cannot be answered in the present 
state of the science. The method of oscillations, which Cou- 
lomb employed, and which we have explained, is the most deli- 
cate and simple of all known means of discovering the presence 
of iron in the products of nature and art, even when it exists in 
exceedingly small quantities. We have only to form needles of 
the substance which we would examine, to make them oscillate 
between two powerful magnets, and to compare their oscillations 
with those of needles made of iron combined with some other 
substance not magnetic, the relative proportions of the iron and 
the unmagnetic substance being known. t And by this method we 
can not only discover and measure exceedingly small quantities 
of iron in its metallic state, but even when it is in the most inti- 
mate combination with oxygen and other substances. I will 
illustrate this remark by an example. Among those minerals 
which have been referred to the class called mica, there is a 
great number whose chemical properties are exceedingly differ- 
ent ; and the laws of the polarization of light, applied to these 
substances denote very different crystalline structures. In the 
course of my inquiries, relative to this subject, an account of 
which is contained in the Memoirs of the Academy of Sciences for 
the year 1816, 1 was led to compare two specimens of mica, one of 
which was brought from Siberia, and the other from Zinwald in 
Bohemia, the latter being mixed with crystals of tin. Although 
the lamina of these two specimens of mica were very transparent, 
chemical tests applied to them, indicated the presence of oxide 
of iron, but in very different proportions. The Zinwald mica 



t For the formulas necessary for this purpose see Biot's Traite de 
Physique, 



262 Magnetism. 

contained hy far the largest quantity ; according to the exact 
analysis of M. Vauquelin, the oxide of iron made 20 hundredths 
of its weight. Proceeding to analyze the other mica, the method 
of trying both by means of magnetism suggested itself to me. 
I then cut a number of small rectangular plates of mica of equal 
dimensions, and fastened them parallel to each other in bundles ; 
and having made these bundles oscillate successively between 
two powerful magnets, suspending them by flattened silk threads, 
whose torsion was wholly imperceptible, I found that the bun- 
dle of Zinwald mica made 12 oscillations in 55", while the other 
bundle made only 7 in the same time. The magnetic forces 
then were as the squares of these numbers, that is, as, 144 to 49, 
Now, if we consider these forces as proportional to the quantities of 
combined oxide of iron, we shall see, that if the Zinwald mica con- 
tains 20 hundredths of this oxide, the other mica must contain 

— - — or 6,8. And the result of the chemical analysis to which I 
144 ' J 

had recourse after this experiment, gave this proportion exactly. 

I do not doubt that, in most cases, this kind of test would be 

found equally useful, and that it would lead to curious results 

respecting the intensity of the combination of iron with other 

substances ; as to its accuracy it cannot be called in question, 

after the experiments of Coulomb, as above stated ; and no one 

can make use of this method without being convinced of the 

truth of what is here advanced. 



Of the Laws of Terrestrial Magnetism in different Latitudes. 

219. We have observed, that the inclination and declination of 
the needle and the intensity of magnetic forces are each dif- 
ferent in different parts of the earth. The processes necessary 
for determining these phenomena have been fully explained. 
We have only to carry a magnetized needle to the different 
places to be examined, or to employ several needles capable of 
being compared with each other, and to observe the three par- 
ticulars above mentioned. Experiments of this kind were per- 
formed about the year 1 700, by the celebrated Dr Halley, to 
whom the English government entrusted a vessel destined to the 



Laws of Terrestrial Magnetism in different Latitudes. 263 

purpose of transporting him and his instruments to different 
regions of the globe. But the researches of Dr Halley being 
directed chiefly to the determination of the longitude by the 
declinations of the compass needle, he confined himself princi- 
pally to observations of this kind, which unfortunately are most 
liable to change ; so that when we now have occasion to speak 
of the state of terrestrial magnetism, it is necessary to have re- 
course to the disconnected observations of more modern naviga- 
tors. But the needles used in these cases being exceedingly dif- 
ferent, as well as the methods of taking observations, it is evident 
that the results must be crowded with seeming anomalies, so that 
at best we can only expect to find confirmations of the most 
general facts belonging to this subject, without being able to 
enter much into detail. In fine, what increases the difficulty, is 
the entire absence of observations throughout a great part of the 
globe, where they are the more needed, as a multitude of facts 
seems to indicate in those parts the action of very remarkable local 
causes, which we are unable to form any conception of, without 
the aid of observation. For these reasons I shall at present con- 
fine myself to what can be gathered from the general aspect of 
the phenomena, without attempting to connect them by the cal- 
culus, for the application of which the most essential data are 
wanting. This will be sufficient to point out to those employed in 
voyages of discovery the regions where it is important for them 
to direct their attention and to multiply their observations. 

220. I will now consider the difference of magnetic inclination 
in different parts of the earth, because this phenomenon seems 
to vary with the time much less than the declination. To dis- 
cover any law relative to the inclination, the point first to be attend- 
ed to, is to ascertain the parts of the globe where it is nothing ; 
that is, where a needle, which, before being magnetized, rested in a 
horizontal position, would remain horizontal, after being magnet- 
ized. A series of such points being connected would form on 
the surface of the earth a curved line, called the magnetic equa- 
tor, and which all authors have hitherto considered as a great 
circle of the earth, inclined to the terrestrial equator at an angle 
of about 12°. Such in reality is the form which the numerous 
observations made on the portion of the magnetic equator, com- 
prehended by the Atlantic ocean, seem to point out. This por- 
tion, being on the route of European vessels destined for America 



264 Magnetism. 

and India, has been more frequently observed than any other. 
The great circle indicated by these observations would cut the 
terrestrial equator at two points or nodes, one of which, the most 
western, would be situated at about 113° 14' of west longitude 
from the meridian of Greenwich ; that is, in the South Sea, near 
the island Gallego, at the distance of nine hundred leagues 
from the coast of Peru ; so that the opposite node would be at 
293° 14' of west longitude. Such has been hitherto the prevail- 
ing opinion. But the above particulars are entirely erroneous, 
so far as regards all those parts of the South Sea, situated above 
the west node, between 113° and 268° of longitude, compre- 
hending, in fact, nearly a hemisphere of ocean. By examining 
the observations made with the utmost care by William Bayly 
and Captain Cook, in two separate vessels, employed in 1777 to 
navigate the South Sea, it will appear that they have each fixed 
the magnetic equator at 156° 30' 9" of west longitude, and at 
3° 13' 40" south latitude ; whereas, by continuing the great cir- 
cle, indicated by observations made in other parts of the earth, 
this equator should have a north latitude of 8° 56' 30". It hence 
appears that the magnetic equator, after meeting the terrestrial 
equator at about 113° of west longitude, descends again in a 
southerly direction ; and, as has been shown by the observations 
of Bayly, confirmed in this particular by those of Dalrymple, 
that there is no inclination in the China Sea, at about 7° of 
north latitude and 254° of west longitude, we must conclude that 
between this longitude and that of 156° 30' west, as determined 
by the observations of Cook, the magnetic equator cuts the ter- 
restrial equator at least once ; and this makes it necessary to 
suppose that it cuts it a second time, near the eastern coast of 
Africa, since we find it again in the Atlantic ocean, with a south 
latitude. So then there are at least three nodes, and perhaps 
four, if the magnetic equator, about its western node, ascends a 
little towards the north before descending to the south near the 
archipelago of the Society Islands. The situation of these nodes, 
and the true form of the line of no inclination between them, have 
been very ingeniously determined by M. Morlet, with the aid of a 
method of interpolation depending upon principles to be explained 
hereafter; and we hence arrive at the curve represented in plate iv. 
221. This curve cuts the terrestrial equator for the first time, 
at about 18° of east longitude, reckoned from the meridian of 



Laws of Terrestrial Magnetism in different Latitudes. 265 

Greenwich, on the western coast of Africa. Thence keeping an 
easterly direction it descends to the south of the equator, from 
which it continues to depart, until it has reached 14° 10' of 
south latitude, this limit being at 26° of west longitude ; it then 
becomes for a short space nearly parallel to the terrestrial equa- 
tor. But leaving this maximum, it gradually ascends towards 
the continent of America, until it reaches a point of about 96° of 
longitude, one hundred and twenty leagues to the west of the Gal- 
lapagos Islands, in the Pacific Ocean ; here we again find it very 
near the equator ; but then the curve is inflected, becoming more 
and more nearly parallel to the equator, and instead of cutting 
it, it approaches so as just to touch it at about 118° of west lon- 
gitude ; after which it descends again to the south, until at 161° 
it reaches a second maximum of south latitude, of about 3° 15', 
on a meridian nearly intermediate between the archipelago of 
the Friendly and that of the Society Islands. On leaving this 
point it descends gradually towards the north, and cuts the ter- 
restrial equator at 184° of west longitude, or 176° of east longi- 
tude, not far from the meridian of the Mulgrave Islands ; then 
continuing its progress to the north, it reaches its first maximum 
of northern latitude, at about 130° of east longitude, near the 
meridian of the Phillipine Islands, where its distance from the 
equator is about 9° ; thence it approaches somewhat nearer the 
equator, and attains a minimum at about 108° of longitude, at 
the entrance of the gulf of Siam, a little to the south of the Isle 
of Condor, where the latitude is not more than 7° 44' north. It 
soon begins to ascend again in a northerly direction, traverses 
the bay of Bengal, cuts the southern extremity of India ; and 
returning to the northern hemisphere reaches its absolute maxi- 
mum of northern declination from the equator, namely, 11° 47', 
in the Arabian Sea at 64° of east longitude. Descending now 
toward the equator it cuts the eastern coast of Africa a little to 
the south of the Straits of Babelmandel ; and traversing the inte- 
rior of the continent to the eastern coast, it returns to the point 
of the terrestrial equator from which we began to trace its 
course. 

222. The magnetic inclinations, observed on each side of the 
line which we have traced, are found to increase as we depart 
from it. If we confine our attention to that part of the globe 
where the magnetic equator seems to be nearly circular, which 

E. * M. 34 



266 Magnetism. 

comprehends Europe, the Atlantic Ocean, and the eastern coast 
of the American continent, it will be seen that the inclination 
remains nearly constant on parallels situated at equal distances 
on each side of this equator ; so that according to this law the 
maximum of inclination would be in two opposite points of the 
earth, the northernmost of which would be found in 23° west 
longitude, and 90° — 14°, or 76° north latitude ; while the other, 
diametrically opposite, would be situated in 203° of west longi- 
tude, and 76° of south latitude. These then would be the poles 
of the magnetic equator, if this equator were circular ; and the 
dipping needle would in these places be vertical. But this is 
not conformable to fact ; for the voyages of discovery, recently 
undertaken by the English in the northern regions, furnish dif- 
ferent results ; very considerable inclinations, exceeding 84°, 
were observed in longitude 61° and latitude 75° ; but the decli- 
nations, which amounted to 87°, were still western, as at London. 
Whence it appears, that the true magnetic pole is further towards 
the west than the preceding conclusions would seem to indicate. 
Nevertheless, when we consider the inclinations only with a view 
to calculating them, either near that portion of the magnetic 
equator which is sensibly circular, or at distances so great that 
its inflexions shall not appear, we find that they can be nearly 
represented by numbers, if we suppose at the centre of the earth 
a very small magnet, or, which amounts to the same thing, two 
magnetic centres infinitely near to each other, exerting an influ- 
ence over every part of the surface of the globe, according to the 
ordinary laws of magnetic forces, that is, in the inverse ratio of 
the square of the distance. A confirmation of this result, derived 
from observation, may be found in a memoir published by M. 
de Humboldt and myself, on the variations of terrestrial magnet- 
ism in different latitudes. If we refer the several parts of the 
earth's surface, in the way of latitude and longitude, to the mag- 
netic equator, considered as a great circle of the earth, we shall 
find by calculation, that for all zones where this circular form is 
admissible, the tangent of depression is double the tangent of 
magnetic latitude.* But unfortunately this simple law cannot be 

* I did not at first enunciate the ratio of the inclination to the 
magnetic latitude under this form, but under one more complicated. 
M. Kraft in an examination of my calculations, published in the Me- 



Laws of Terrestrial Magnetism in different Latitudes. 267 

•xtended without modification to places exposed to the influ- 
ence of the causes by which the magnetic equator is inflect- 
ed. This discordance is unavoidable; for the supposition of 
two magnetic centres exceedingly near to each other, and situ- 
ated at equal distances from the centre of the globe, necessarily 
implies, that the magnetic equator is strictly a great circle; a 
circumstance which the observations above mentioned wholly 
forbid our admitting. So when we attempt to apply the rule 
founded on the relations of the tangents, to some of the islands 
of the South Sen, to Otaheite, for instance, where Cook took so 
many observations, we find the southern inclinations much too 
great ; while, on the other hand, the inclinations calculated for 
countries situated south of the American continent, at about the 
same longitude, are much too small. These irregularities neces- 
sarily result from the inflection, by which, in this part of the 
globe, the magnetic equator approaches the south pole, and they 
afford a striking confirmation of these inflections.* We find 
like irregularities when we attempt to apply the law of the tan- 
gents to observations made in India. 

It is necessary then, in order to satisfy the conditions involved 
in these phenomena, to suppose near the archipelago of the 
South Sea some disturbing cause, as a particular centre of mag- 
netic force, exerting an influence chiefly on this hemisphere, 
and modifying the central action. Indeed, this supposition re- 
conciles all our results, and requires only an inconsiderable 
secondary central force, deriving its energy almost entirely from 
its proximity. 

223. It is probable that similar and equally slight modifica- 
tions will enable us to account for the phenomena observed 
in the Indian Ocean. But before we attempt to determine the 
centres of these purturbations, or to estimate their influence, we 

moires de Petersburg, for 1809, inquired whether a more simple 
enunciation might not be derived from observations merely, consid- 
ered as empirical ; and it was in this way that the enunciation given 
above suggested itself to him. But he afterward perceived, that it 
was only a very simple transformation of mine ; and I have ^availed 
myself of this happy remark. 

* The existence of the inflection, so often mentioned, has been 
also verified by Captain Freycinet, in his voyage round the world. 



268 Magnetism* 

must study the variations in the declination of the magnetic 
needle, and the intensity of the magnetic forces, as observed ia 
different latitudes. For as these phenomena result, in like man- 
ner, from the magnetic action of the globe, they must be taken 
into consideration, in the attempt we may make to represent this 
action. 

As, in our inquiry respecting the magnetic inclination, our first 
object was to find the series of places where it is nothing ; so 
in examining the phenomena presented by the declination, we 
must begin by determining the points on the globe where 
it is nothing, and which continued, would form a curve called 
the line of no declination.* These lines do not take the direc- 
tion of geographical meridians ; they are, on the contrary, 
very oblique to these lines, and they present very irregular in- 
flections. According to the latest observations, there is now a 
line of no declination in the Atlantic Ocean between the old and 
new world. It cuts the meridian of Paris at about 65° of south 
latitude ; thence it ascends to the northwest, about 33° of 
longitude, w ? here it may be traced on the heights of the coast 
of Paraguay; after which, acquiring again a direction nearly 
north and south, it passes along the coast of Brazil, and thus 
reaches the latitude of Cayenne. But then suddenly shifting its 
direction to the northwest, it directs itself towards the United 
States, and thence towards the other northern parts of the 
continent of America, which it traverses without altering its 
direction. 

224. This line is not fixed on the globe ; at least for a century 
and a half, it has had a considerable motion from east to west. 
In 1657, it passed through London, and in 1664 through Paris; 
so that according to its present direction, it has traversed on this 
parallel of latitude [Paris] nearly 80° of longitude, in the course 
of 1 50 years. But it seems evident, that this motion is not uniform ; 
it is even unequal on different parallels ; for, in the Antilles, for 
example, the declination has scarcely undergone any change 
for 140 years. And, in general, when we consider how very 
slow this motion is, we are not by any means certain that it is 



* The substance of this discussion was furnished me by M. de 
Humboldt. 



Laws of Terrestrial Magnetism in different Latitudes. 269 

always progressive, or that it will continue in any particular 
direction. The very careful observations which have been regu- 
larly made in the observatories of England and France, have 
seemed to indicafe, for some years, the commencement of a retro- 
gradation towards the east ; but a like retrogradation was observ- 
ed in the years 1790 and 1791, which did not continue. Time 
only can make us fully acquainted with these phenomena. 

225. The very exact determination of the inclination, as ob- 
served at different times, by Gilpins and Cavendish at London, 
proves that this element also is variable, although much less so 
than the declination. The inclination in 1775 was 72° 30' ; in 
1805, 70° 21'. This result has been confirmed in France by 
the experiments of M. de Humboldt. It is proved in a still more 
striking manner by the successive observations of the inclination, 
which different navigators have made from the year 1751 to 
1792, at the cape of Good Hope, and which show, that during 
this interval there has been a progressive increase of inclination 
which now amounts to 5°. 

There is another line of no declination, nearly opposite to the 
one just described ; this, constantly directing itself to the north- 
west, takes its origin in the Southern Ocean, cuts the western ex- 
tremity of New Holland, traverses the Indian Ocean, strikes the 
continent of Asia at Cape Comorin, and thence traversing Persia 
and the western part of Siberia, ascends towards Lapland. But 
what is \ery remarkable, this line becomes forked near the 
Asiatic archipelago, and gives rise to another branch which, di- 
recting itself almost exactly north and south, passes through 
this archipelago, traverses China, and enters the eastern part 
of Siberia. The two branches which proceed from this line, 
either have no motion, or an exceedingly slow one. It seems 
that there has been no sensible change in the declination at New- 
Holland for the last 140 years. 

Traces of a fourth line of no declination were observed by 
Cook in the South Sea, near the point of the greatest inflection 
of the magnetic equator. Navigators have not followed these 
indications of the line to the northward, but it is extremely pro- 
bable that it is continued ; for, as has been very justly remarked 
by M. de Humboldt, since the declination changes its algebraical 
sign from west to east, or from east to west, in passing from one 
side of each line of no declination to the other, it is necessary, 



270 Magnetism. 

taking in the whole globe, that the number of lines of no declina- 
tion should be equal, so that after all the changes from plus to 
minus and vice versa, we return to the sign from which we set out. 

226. Having determined the direction of the lines of no decli- 
nation, it is necessary in order to limit these phenomena in an- 
other respect, to enumerate the places where this declination is 
greatest. With respect to this particular also we discover very 
irregular lines, which fall between those just mentioned. The 
greatest declination observed in the southern hemisphere by Cook, 
was at 60° 40' of latitude, and 91° 25' of west longitude from the 
meridian of Greenwich ; this was 43° 45'. In the northern hem- 
isphere as we can approach much nearer to the magnetic pole, a 
much greater declination has been observed, in some cases ap- 
proaching to 90°. Such are those observed in the English ex- 
peditions to the north pole. The numerous compass needles, 
which in our climate direct themselves towards the north Were 
here turned to the west. They ought even to direct themselves 
to the south if we pass the magnetic pole ; and the direction of 
the needle would become wholly indeterminate upon arriving at 
the pole itself, since, the resultant of magnetic forces being then 
vertical, its horizontal element would be nothing. In general, it 
is evident from this reasoning, that the horizontal directive force 
must be quite feeble in places where this inclination is very 
great ; so that if the smallest foreign force intervenes, whether of 
the ferruginous substances situated near the earth's surface, or of 
the iron used in the construction of vessels, it must exert a very 
decided influence over the compass needle, and almost entirely 
neutralize its directive power. Such is undoubtedly the expla- 
nation to be given of those singular and unexpected variations 
and irregularities, which take place in the direction of needles in 
high latitudes, as formerly observed, and now more recently by 
the English. 

227. After having thus related all that is at present known on 
the subject of the direction of magnetic forces in different parts 
of the earth, it only remains to consider the absolute intensity of 
these forces. This subject has been much less studied than that 
of the declination and inclination ; undoubtedly on account of 
its being attended with more difficulty. The first correct obser- 
vations on the intensity were made by M. de Humboldt, in his 
extenstive travels, and by M. de Rossel, in the expedition of Admi- 



Lams of Terrestrial Magnetism in different Latitudes. 271 

ral Entrecasteaux. Very valuable information relating to mag- 
netic intensity may be learned from Captain Freycinet's voyage 
round the world, and from the English expeditions to the North 
Pole. 

We are indebted to MM. de Humboldt and de Rossel, for the 
discovery of a very remarkable phenomenon already referred 
to, namely, the general increase of magnetic intensity as we pro- 
ceed from the equator towards the poles. 

The same compass needle which, at the departure of M. de 
Humboldt, made at Paris 245 oscillations in 10 minutes, made at 
Peru only 21 1, as we have already mentioned ; and it has always 
been found that the number of oscillations diminishes as we 
approach the magnetic equator, and increases as we depart from 
it north or south. We cannot attribute these differences to a 
diminution of magnetic force in the needle, nor can we suppose 
that it is materially affected by time or heat ; for in the case of 
M. de Humboldt's needle, after having remained three years in 
the hottest regions of the earth, it gave a second time, at Mexico, 
oscillations as rapid as at Paris. In fine, M. de Humboldt has 
spared no pains to render his observations accurate ; and they 
are confirmed by the results obtained from making needles oscil- 
late successively in the magnetic meridian, and a plane perpendic- 
ular to this meridian. Indeed, the inclination deduced in this way 
is found by M. de Humboldt to accord with that obtained by 
direct observation, although he was not at the time aware, of the 
relations subsisting between his elements which M. Laplace has 
since pointed out. As the accuracy of these observations cannot 
be called in question, we must also give our assent to the conse- 
quence which results from them, namely, that the increase of 
magnetic terrestrial force is constant from the magnetic equator 
to the poles. The experiments made by M. de Rossel at Brest 
and in New Holland, also lead to the same conclusion. 

228. The account w T hich we have given of the present state 
of our knowledge respecting the magnetism of the globe will 
serve to show our imperfect acquaintance with this subject. 
And ignorant as we are of a great many necessary data, espe- 
cially of such as relate to the magnetic declination, we cannot 
expect to discover the real causes of these phenomena. All that 
we can do is to inquire into the empirical laws, which, while they 
embrace the greatest possible number of facts, represent their 



272 Magnetism. 

numerical relations, and indicate the principal elements, with 
respect to which it is necessary to appeal to observation. I have 
already remarked, that most of the observed inclinations, espe- 
cially in those parts of the globe where the inflections of the 
magnetic equator are hardly sensible, can be represented very 
nearly by the action of two magnetic centres, placed at a small 
distance from each other, near the centre of the earth. M. 
de Humboldt and myself were led to this result, in the course 
of an investigation of which I have spoken above ; and our me- 
moir was already published, when I learned that the celebrated 
astronomer Mayer had previously arrived at the same conclu- 
sions, while discussing the subject of the inclinations known in 
his time, and that he had availed himself of a like method of 
representing the declinations in a memoir read before the Society 
of Gottingen, but never printed. The son of this great astrono- 
mer, having politely favoured me with an extract from the above 
memoir, I have satisfied myself of the agreement of our ideas ; 
1 have also learned, that Mayer discovered, by means of experi- 
ments, the law of magnetic attractions, namely, that they are 
inversely proportional to the square of the distance. 

This common conclusion, deduced from elements so various, 
seems to indicate something more than a law purely empirical. 
We ought then to subject it to a stricter examination. It is 
easily seen on a cursory view, that a single magnet, placed at 
the centre of the earth, would not fully explain these phenomena ; 
for on this supposition the magnetic equator must be a great cir- 
cle perpendicular to a straight line drawn through the two cen- 
tres of action ; and this would not account for any of those 
inflections which the curve of no inclination actully presents. 
Besides, such a magnet, however we suppose it placed, would 
necessarily give corresponding phenomena on the two sides of 
the plane drawn through the two centres of action and the centre 
of the earth, a correspondence by no means conformable to facts 
as actually observed, especially in the South Sea, and on the 
continent of Asia. 

Being unable then fully to adopt this simple theory, let us 
adhere to it as closely as possible ; and having found that it 
affords a sufficient explanation of the observations made in Eu- 
rope and in the Atlantic Ocean, let us try the effect of a modifi- 
cation, which shall have very little influence in this quarter of 



Laws of Terrestrial Magnetism in different Latitudes. 273 

the globe, and a very considerable one in the opposite, where 
the magnetic equator suddenly undergoes its most remarkable 
inflection. I refer to that portion of the Pacific which is in ahout 
161° of longitude. This modification consists in supposing in 
these regions a second excentric magnet, whose position and re- 
lative power may be so adjusted, as to satisfy the known ob- 
servations. But in performing the calculation, we find that it 
is only necessary to give to this magnet a very feeble force, in 
order to explain all those anomalies which occur on this side of 
the globe, and to reconcile the very small inclinations observed 
in the southern part of the South Sea, with the large ones observ- 
ed in the northern parts of the continent of America. By dis- 
tributing in this manner other secondary centres in those parts 
of the globe where the irregularities of the declination seem to 
be most striking, it is very probable that we might succeed in 
giving an accurate representation of these, as well as of the in- 
clinations and intensities. It is thus, that in the solar system the 
principal motion, which is that caused by the attraction of the 
sun, is modified by the disturbing forces produced by the small 
masses of the planets. But as it is necessary to know the places 
of these planets, in order to calculate their influence, so it is 
necessary that the places of these secondary centres should be 
indicated by accurate observations before we can calculate their 
effects. 

229. Is this central magnetic action, which so many phenom- 
ena lead us to suppose, really produced by a magnetic nucleus 
enclosed in the centre of our globe, or is it the principal resul- 
tant of all the magnetic particles diffused through its mass ? 
No decisive answer can be given to these questions; although 
the last supposition seems the more probable. In this case, the 
secondary centres would be situated in places where some local 
attraction happened to be preponderant. And in reality, obser- 
vations have incontestibly shown, that the general system of in- 
clinations, declinations, and of magnetic intensities, is very per- 
ceptibly modified, and sometimes in a very sudden and irregular 
manner, by the vicinity of large chains of mountains. This 
seems to be confirmed also -by the singular inflection which the 
magnetic equator undergoes near the numerous archipelagos of 
the South Sea. We know, indeed, that the islands with which 
this sea is studded, are only the summits of very high mountains, 

E. & M. 35 



274 Magnetism. 

which raise themselves in peaks from the bosom of waters never 
jet fathomed. If the madrepores of which they seem to be com- 
posed, form only a thin stratum, and if, as some very ingenious nat- 
uralists have supposed, the rest of their mass has been produced 
by, or subjected to the action of subterranean fires, the range of 
these islands would form the most extensive volcanic chain in 
the known world. Then the irregularities, thus produced in 
the general laws of terrestrial magnetism, would be entirely con- 
formable to what we observe in all volcanic countries. For the 
action of subterranean fires mu&t necessarily change the chemi- 
cal state and natural arrangement of the ferruginous matter in 
those places where it is exerted, and thereby disturb the direc- 
tion of the magnetic needle, and to a certain extent modify the 
general action of the globe. We have, indeed, several examples 
of such variations which have happened suddenly. M. de Hum- 
boldt observed a phenomenon of this kind at Peru, after a vio- 
lent earthquake. It is possible, then, that the secondary centre 
in the South Sea may be referred to causes of this nature. Anal- 
ogous ones may undoubtedly be found in other countries ; and, 
as there have taken place, within the last two hundred years, 
variations in the declination of the compass needle, so extraordi- 
nary and irregular, that down to the present time, it has been 
found impossible to reduce them to any law, does not this very 
irregularity seem to indicate the operation of some variable and 
inconstant cause ? According to this supposition we have no rea- 
son to expect in Europe the return of the needle to an eastern 
direction ; and indeed, since it has ceased to move in a westerly 
direction.no sensible retrogradation has been noticed; so that 
while we have only the present observations for data, it is impos- 
sible to decide whether it will ever revert to its former positions. 
230. This system of the distribution of magnetic forces, to 
which our inquiry into the'inclinations has led us, is singularly 
confirmed by the remark, that the law of the tangents, which 
results from the action of two centres infinitely near to each 
other, is peculiarly applicable to all those phenomena, which we 
have undertaken to discuss. For example, we find striking 
proofs of it in the method of interpolation, by which M. Morlet 
succeeded in determining the true form of the magnetic equator, 
of which I have given an account above. Indeed, he did not 
arrive at this important result by the aid of new observations 



Laws of Terrestrial Magnetism in different Latitudes, 275 

taken in places where there is no inclination ; he discovered it in 
the course of an investigation very ingeniously conducted by 
means of observations already known. A great number of navi- 
gators have traversed the magnetic equator ; but very few have 
determined by observation the precise point where the needle 
is exactly horizontal. They have only observed at places north 
or south of the equator, where the inclination is very small; and 
observations of this kind are but too few even at the present 
day. (t is evident then, that in order to determine the magnetic 
equator more acurately than has yet been done, we must dis- 
cover some means of ascertaining it from observations taken in 
distant places, or at least in places more distant, than those which 
have until now been resorted to. This object M. Morlet effected 
in the following way. Let us suppose, that we have in some 
place observed an inconsiderable inclination of the magnetic 
needle. This point will of necessity be but a short distance 
from the magnetic equator. Suppose that we have also deter- 
mined the declination ; or that by means of a series of lines of 
declination near the above point we have discovered the direc- 
tion of the magnetic meridian. This being continued will inter- 
sect the magnetic equator, and its distance from the point in 
question will be measured by the arc of a great circle situated 
in the plane of the magnetic meridian. This being established, 
M. Morlet considers the distance above mentioned as an instance 
of magnetic latitude in the system of two centres, and he de- 
duces it from the condition that its trigonometrical tangent is 
half the tangent of the observed inclination. 

The object of an experimental law being to connect together 
phenomena, it must be admitted as soon as this object is attained, 
whatever be the nature of the speculative ideas by which we 
arrive at it. The rule of M. Morlet, purporting to be simply a 
method of reduction and interpolation, is to be estimated by the 
results to which it leads. It may be proved in two ways ; first, 
by choosing places where the magnetic equator has been deter- 
mined by observations taken on the spot, and seeing if the rule 
applied to observations at distant places gives the same points. 
The other consists in determining each point of the magnetic 
equator by a great number of distant observations, reduced 
according to this rule, and observing if they all agree in as- 
signing the same position. These two modes of verification 



27G Magnetism. 

have been applied by M. Morlet to numerous observations ; 
and the results agree with surprising accuracy, thus confirming 
the truth of the method of reduction so ingeniously devised. 

231. We may hence deduce a very important consequence. 
Since, near the magnetic equator, the tangent of inclination is 
always double the tangent of the magnetic latitude, reckoned on 
the magnetic meridian, it follows, that under these circumstances, 
the magnetic needle directs and inclines itself precisely as it would 
do, if it were attracted by two magnetic centres infinitely near 
to each other, and situated at a great depth below the earth's 
surface, and in the direction of the perpendiculars drawn through 
the several points of the magnetic equator ; in other words, all 
the forces that determine the direction of the needle, are so com- 
bined as to produce a result, which must, according to our pres- 
ent limitations, be considered as emanating from two similar 
centres. 

Undoubtedly this result will be only an approximation to the 
truth. If, as is probable, the direction of the needle is really 
the effect of a principal central force, combined with much 
smaller secondary forces, the resultant of all the forces cannot, 
strictly speaking, resolve itself into the mere action of two cen- 
tres, varying according to the square of the distance. But, for 
a small angular extent, and for certain positions about the cen- 
tres of the forces, it is possible that this reduction may be suffi- 
ciently accurate. M. Morlet also discovered, that his rule is 
only applicable to certain amounts of inclination, which are not 
the same under different meridians, and each side of the mag- 
netic equator; but w r hich, in every case, altogether exceed the 
limits within which it would have been necessary to restrict 
thetn, had we confined ourselves to an arbitrary method of inter- 
polation less intimately connected with the secret cause of the 
phenomena. 

232. The magnetic action of the globe is not confined to its 
interior, or to its surface." It extends above the surface of the 
earth, as is shown by the experiments of M. Gay-Lussac and 
myself, performed by means of a balloon. It appeared, more- 
over, from our observations, that the intensity of this action, like 
that of gravity, continues nearly the same for small distances as 
we ascend from the surface of the earth, for we did not find any 
sensible diminution at the elevation which we attained. Its dimi- 



Laws of Terrestrial Magnetism in different Latitudes* 277 

nution probably follows the general law of magnetic attractions, 
namely, the inverse ratio of the square of the distance ; and thus 
it seems to extend indefinitely into space. Analogy would lead 
us to think that the moon, the sun, and the other heavenly-bodies 
are indued with like forces. The magnetic action of all these 
bodies ought then to exert an influence, according to their posi- 
tions and their distances, on the direction of the magnetic needle, 
as well as on the absolute intensity of the magnetic directive force ; 
and as these positions and distances are changing continually, on 
account of the earth's motions and those of the planets, perpetual 
variations are to be looked for in the magnetic resultant. If, for 
example, the magnetic action of the sun and moon be sensible, 
the earth's motion on its axis and in its orbit, must be attended 
with annual and diurnal oscillations of the magnetic needle. But 
we have not only full proof, that there really are such motions, 
but their periods, determined by a long series of observations, 
agree with the cause to which we have referred them. At Paris, 
according to M. de Cassini, the maximum of diurnal declination 
occurs between noon and three o'clock in the afternoon, when 
the needle is stationary ; it then approaches toward the terres- 
trial meridian till about eight o'clock in the evening ; from which 
time it ceases to change its position, remaining stationary during 
the nia;ht. The next day, at about eight o'clock in the morning, 
it recommences its motion from the meridian. If this second 
departure exceed the first, we infer that the declination is in- 
creasing from day to day ; in the contrary case, it is supposed to 
be diminishing. The greatest diurnal variations usually take place 
during the months of April, May, June, and July ; that is, between 
the vernal and autumnal equinox. At Paris, they vary from 
13' to 16'. The smallest are from 8' to 10 7 ; and they take place 
during the remainder of the year. Now if we compare similar 
situations of the needle on different days, and at corresponding 
hours, in order to determine its general course, we shall find that 
from the vernal equinox to the summer solstice, the north pole 
of the needle inclines towards the east, and that it tends 
westward the rest of the year, that is, from the summer sol- 
stice to the vernal equinox. For the knowledge of these 
periods we are indebted to M. de Cassini, who deduced them 
from eight years' observations, made at the observatory in Paris. 



278 Magnetism. 

233. It appears, moreover, by numerous observations, that 
the magnetic needle is subject to sudden and irregular variations 
at the time of the luminous meteor, called the aurora borealis. 
These .variations are frequently of but short continuance ; that 
is, after the needle has been thrown into rapid agitations, during 
the appearance of the meteor, it resumes its ordinary position, 
and recovers its wonted motions ; but it sometimes happens, that 
the deflection is permanent. It has also been rem rked, that 
there are instances in which the needle is apparently under the 
influence of the meteor, when no meteor is to be seen at the 
place where the phenomenon occurs. But in such cases we 
always find, that the meteor has presented itself with more or 
less distinctness, either at the same moment or a few hours be- 
fore or after, in some countries farther. to the north or south ; so 
that these unusual agitations of the needle may be considered 
as a proof of the existence of the meteor, and may perhaps be 
regarded as the precursor of it. The interesting nature of these 
phenomena, and the difficulty of accurately observing in central 
Europe the meteor which seems to occasion them, have led me 
to give, in this place, a particular account of the circumstances 
that attend it. 

234. The aurora borealis appears in the night at irregular 
intervals, extending itself along the northern part of the heavens, 
now as an indefinite faint light, rising a little above the horizon 
and resembling the twilight ; now as phosphoric confiscations, 
suddenly traversing and illuminating the whole atmosphere. 
These luminous appearances were for a long time the only cir- 
cumstance that engaged the attention ; but in 1740, two Swedish 
observers, Celsius and Hiorter, discovered other and entirely 
new phenomena in this meteor, which being intimately connect- 
ed with its nature, very much extended the views which had 
been previously entertained upon this subject. They observed, 
that during the appearance of the aurora borealis, magnetic 
needles, freely suspended, almost always undergo very irregular 
agitations, which needles not magnetic, those of copper, for in- 
stance, do not exhibit. If we compare observations of this 
kind made at places very distant from each other, as at Upsal 
and London, for instance, we find that the motions are the same. 
It appears, also, that their violence depends on the brightness 
and extent of the aurora boreaiis. A low and laint glimmering, 



Aurora Borealis. 279 

towards the northern horizon ordinarily produces only a very 
slight, and perhaps insensible, disturbance of the magnetic 
needle. Pfloreover, the motion is very slight in the case of an 
elevated meteor when the principal focus is situated in the plane 
of the needle's direction, usually called the plane of the magnetic 
meridian. We remark further, that when the phosphoric jets 
are numerous, the atmosphere at the same time being calm, or 
only agitated by a steady breeze, we almost always observe 
that the substance of the meteor is disposed in one or several 
concentric arcs, resembling those of the rainbow, now white, and 
now tinged with the brightest colours. But we almost always 
find, that the common centre of these arcs and their summits 
are situated in the magnetic meridian of the place where they 
are observed, so that they are all similarly situated with respect 
to this plane ; and this coincidence with the meridian, which still 
exists, has been remarked ever since any accurate observations 
were made, although during this time there have been very con- 
siderable variations in the direction of the magnetic meridians in 
Europe; so that the mean direction of the meteor in the horizon 
of each place, has also undergone an equal change. Further- 
more, it sometimes happens, that the phosphoric fires, breaking 
forth from all parts of the horizon, from the east, the west, and 
the north, ascend, or seem to ascend, vertically over the head of 
the observer, even to his zenith, and having passed this point, 
they form by their union a brilliant crown, whose centre is 
situated some degrees lower, near the south east, at least in all 
places where this remarkable modification of the phenomenon 
has been observed. But if we determine the apparent position 
of this crown, either by the aid of astronomical instruments, or 
by observing what stars are comprehended within it at the time 
of its formation, we shall find that its centre, in every place 
where it has been observed, is always situated exactly in the 
direction of that point in the heavens, to which the magnetic 
needle is directed, when suspended by its centre of gravity, in 
such a manner as to admit of its taking its position freely, in 
obedience to the resultant of the magnetic forces exerted upon it 
by the terrestrial globe. I have myself had an opportunity of 
verifying most of the particulars here mentioned in the case of a 
very large aurora borealis, which was visible on the 27th of 
August, 1817, during my visit to the Shetland Islands. 



280 Magnetism 



We first saw in the northeastern parts of the horizon several 
slender jets of light which, having attained a little elevation, con- 
tinued to shine for some time and then vanished; but in about an 
hour and a half afterwards they re-appeared in the same region 
of the heavens, and were now much stronger, more brilliant, and 
more extended. Very soon a regular arc resembling a rainbow 
began to present itself just above the horizon. It was at first 
incomplete, but gradually increased ; and after some moments, I 
saw the other part approaching from the west, and upon being 
formed, it ascended instantaneously, accompanied by a multitude 
of jets of light which rushed towards it from all parts of the 
northern horizon; then the summit of the curvature rose al- 
most to the zenith. This arc was at first wavering and unset- 
tled, as if its component parts had not taken a stable position ; 
but very soon the agitation entirely ceased, and it remained in 
undisturbed beauty for more than an hour, having only a pro- 
gressive motion, and that almost insensible, towards the south- 
east, whither it seemed to be carried by a gentle north-western 
breeze that was then blowing. So that I had sufficient time to 
examine it, and to fix its limits and position with the cin le, used 
in my astronomical observations. I found that it comprehended a 
portion of the horizon amounting to 128° 42', and that its centre 
was situated exactly in the direction of the magnetic needle. 
The whole region of the atmosphere embraced by this arc in the 
north-western part of the heavens, was incessantly traversed in 
all directions by luminous jets, whose different forms, motions, 
colours, and durations, engrossed my imagination no less than my 
senses. Most frequently, each jet at its first appearance was a 
mere stream of whitish light ; its size and brightness rapidly 
increased, and it occasionally presented some very singular va- 
riations of direction and curvature. When completely develop- 
ed, it contracted into a slender rectilineal thread, for the most 
part exceedingly brilliant, and tinged with a very deep red col- 
our. After this it grew fainter and fainter till it finally vanished, 
often at the very place where it first appeared. The long con- 
tinuance of many jets in the same apparent place, considered in 
connection with the infinity of shades assumed by them, seems 
to prove, that the light is not reflected, but direct, and that it is 
actually developed in the place where it is first seen ; besides, I 
have not been able to discover in it the least trace of those phy- 



Aurora Borealis, 281 

sical properties which characterize reflected light ; and which 
are designated by the term polarization. All these fires and 
even the arc which comprehends them, occupy a region more 
elevated than the clouds ; since the clouds themselves intercept 
them; and the edges of these clouds were actually or seemed to 
be tinged with light. The moon, which had then reached a con- 
siderable elevation above the horizon, shed her lustre also on 
this imposing scene, and the tranquillity of her silver light form- 
ed a most agreeable contrast with those vivid corruscations with 
which the atmosphere was inundated, 

235. Having now given a view of the principal circumstances 
attending this phenomenon, we propose to deduce from them the 
conditions of its existence ; and the first thing to be determined 
is, whether it exists in our atmosphere or beyond it. There is 
a simple method of settling this question. If it be beyond the 
atmosphere, it must be independent of the diurnal rotation of the 
earth ; and therefore its jets of fire, its arcs, its luminous crowns 
ought to follow the general course of the stars from east to west, 
and to seem like them to turn about the celestial poles. On the 
contrary, if the meteor belongs to our atmosphere it should par- 
take of the common motion which the rotation of our globe com- 
municates to all terrestrial bodies, and even to the clouds •, it 
should then appear to be immovable with respect to these bodies, 
or at least to undergo only accidental disturbances like the 
clouds themselves. All observations unite in establishing the 
latter supposition ; and the length of time during which the me- 
teor, observed by me at the Shetland Islands, continued, would, 
if necessary, afford a fresh confirmation. 

We may then consider it as an established fact, that the phe- 
nomenon of the aurora borealis takes place in our atmosphere. 
But, as is well known, elevated objects when seen at a distance 
through the atmosphere, are apt to produce many optical illu- 
sions. For example, all the stars seem to us attached to the 
concave part of the same spherical surface or dome ; although 
their distances are infinitely various. The vast trains of lumi- 
nous vapour which form the tails of comets, seem also to apply 
themselves to this dome, although in reality they stretch into 
space in rectilineal directions. By another illusion, when the 
sun is partially concealed behind a mass of clouds, and emits 
rays of light through the openings of these clouds, the rays. 

E. <fr M, 36 



282 Magnetism. 

although actually parallel, appear to converge towards the point 
of the heavens where the sun is. These general laws of per- 
spective must affect, in like manner, the appearance of the lumi- 
nous jets emitted by the meteor in question, and must be taken 
into consideration in our attempts to explain them. But from 
whatever situation these jets are observed, they always seem to 
describe arcs of great circles on the celestial dome, and to con- 
verge towards that part of the heaveans to which the needle points 
when perfectly free. Whence we conclude, that they are in 
reality cylindrical, and parallel to the direction of the needle. 
But each jet presents, moreover, great varieties of size and lustre, 
from which we are led to believe that they are, in fact, composed 
of a great number of shorter cylinders independent of each other, 
and in part piled one above another. As these indications are 
noticed throughout the whole region of space where the meteor 
is visible, we may conclude, with geometrical rigour, that it con- 
sists of a forest of luminous columns, all parallel to the result- 
tant of the magnetic forces, and of course for short distances 
parallel to each other, and suspended at nearly equal heights on 
different sides of the horizon. These columns being situated at 
different distances from the observer, must, by the perspective 
effect, appear to be raised to different heights. They must also 
mutually cover each other, and appear to project one over the 
other, especially when, being seen near the horizon, the visual 
rays proceeding for m them are nearly perpendicular to their 
length ; but after attaining such an elevation that their interme- 
diate spaces may be seen, they must appear to separate ; if then a 
certain number of them be simultaneously transported over the 
head of the observer, in such a manner as to pass by the point 
of the heavens to which the magnetic needle, parallel to them 
directs itself, the projection of all these columns on the celestial 
dome, will form about this point a luminous crown the divergent 
rays of which will seem to descend on all sides toward the hori- 
zon, till they arrive at the apparent height at which the meteoric 
columns will have descended by the effect of the progressive 
motion. 

This constitution of the meteor, which has been deduced 
from optical considerations, is rendered probable by many cu- 
rious facts, which different observers have had occasion to no- 
tice, and which have a relation to the positions which these dif- 



Aurora Borealis, 283 

Cerent parts of the meteor happened accidentally to have with 
respect to them. 

For example, when the meteoric colonnade, already illuminat- 
ed, is situated entirely in the horizon exactly north of the obser- 
ver, if it happens to be transported in a southerly direction, and 
in consequence to approach the observer, without any disap. 
pearance, or change of arrangement, of the columns composing it, 
we ought to expect the same optical effect which is presented by 
the trees of a forest when we approach them ; that is, the col- 
umns situated eastward will separate toward the east, and the 
columns situated westward of the plane of the magnetic meridian, 
will appear to separate toward the west, while those which are in 
this meridian will appear to be stationary, or at least only to ascend 
directly towards the zenith. This appearance was attentively ob- 
served by F. C. Mayer, at Petersburg, in a large aurora borealis, 
which was seen on the 16th of Sept. 1726. I will quote his very 
languaue, observing that by the word " trabs," he designates a 
vertical jet, or one of our luminous columns. He first describes 
the formation of an arc, whose summit was not directed exactly 
to the north, but which had a very considerable declination to 
the west. He then adds, " Motus trabium mirus erat ; quae enim 
in occidentali arcus parte extabant, versus occidentem fereban- 
tur; ad orientem ferebantur, qua? in orientali arcus parte sitae 
erant ; boreales autem trabes stabant immobiles. Ex hoc pha> 
noraeno intellexi lucem moveri ex nord-west versus verticem 
meum, id quod sequentibus phaenomenis confirmatum est." It 
will be seen that Mayer has deduced precisely the consequences 
which are required by the rules of perspective. 

236. Another case which may sometimes present itself, al- 
though very rarely, occurs when the illumination of the meteoric 
colonnade, seemingly accidental, appears for some time to take 
place only over a certain number of the columns which compose it. 
Then if these columns are placed at sufficient distances from each 
other, we may have an opportunity of examining them singly. 
This opportunity was afforded by the remarkable aurora borea- 
lis of 1716, an account of which may be found in the memoir of 
Dr Halley, (Phil. Trans. 347, p. 411, 415.) Small columns of 
equal lengths and parallel to each other were distinctly seen sepa- 
rate in a portion of the heavens surrounded by two luminous and 
almost horizontal belts. An account of a like phenomenon may 



284 Magnetism, 

also be found in another memoir of Dr Halley, (Phil. Trans. No. 
363, p. 1099, lor the year 1719.) He there relates, that from 
time to time, there appeared in the air at a great height collec- 
tions of columns, or co-ordinate luminous beams, resembling 
the pipes of an organ, which presented themselves to view as 
suddenly as if a curtain had been drawn from before them. 
Indeed, if any one will undertake to read the numerous ac- 
counts of this meteor which have been furnished by those 
who have visited the northern regions, he will find a mass of 
facts which perfectly answer to the constitution of the meteor as 
deduced by us from the laws of perspective, and he will not 
meet with any thing opposed to our conclusions. A full state- 
ment of these geometrical deductions has been given by Dalton, 
probably without being aware that they had been already ob- 
tained by Cotes, in 1716, the person of whom Newton said, that 
u if he had lived, we should have known something ;" and that 
they had since been adopted by Cavendish, the most severe and 
cautious of all philosophers. I have made this remark in order 
to show that they may be regarded as rigorous. 

237. After having given a general description of the meteor, 
one of the most essential circumstances to be determined is its 
elevation. Attempts have been made without number to ascer- 
tain this point, by the aid of the same processes which geometry 
affords for measuring the distances of inaccessible objects ; that 
is, by observing in different places, at the same time, the position 
of the same part of the meteor. But the difficulty of obtaining 
this perfect identity as to time and point of the object, renders 
the application of the method very uncertain ; and accordingly 
the results obtained by it assign to the meteor uncertain heights, 
varying, in some cases, from twenty to more than one hundred 
leagues. Still more uncertainty prevails with respect to the 
length of the meteoric columns, which some have attempted to 
measure by like processes. If, in fine, the estimates made under 
certain favourable circumstances appear worthy of confidence, it 
may be urged, I think, that they are not general ; and that, in 
certain cases at least, the meteor descends much lower than we 
should thus be led to suppose. This seems probable from the 
quick and continual agitation of the phosphoric jets, the simultane- 
ous progressive motions of the arcs, like that which a gentle breeze 
might be expected to give them, the slow and regular transfer of 



Aurora Borcalis, 285 

those fleecy portions of phosphoric matter, which travellers in 
the northern regions assure us they have often seen floating 
separately in the atmosphere ; and I myself saw a like phenom- 
enon at the Shetland Islands, the 6th of September, 1817. It 
was a dense cloud which slowly ascended above the horizon 
from the north-east. Its sides were the centres of a phosphoric 
light which seemed at one time to remain behind till it was ex- 
tinguished, at another, to break forth and illuminate the edges of 
the cloud. I can give no better idea of this phosphorescent ap- 
pearance, than by comparing it to the dark clouds of our theatres 
when illumined by lamps from behind. Yet for some moments I 
observed on its inferior surface a small spot where the light seem- 
ed to intervene between it and me. This cloud, having attained a 
height of about 45°, remained for some time stationary, and then 
gently moved to the west, still retaining its phosphorescence ; some 
jets of light also, proceeding from the northern horizon, inclined 
towards the west, as if a wind in the higher regions of the atmo- 
sphere, coming from the south-east, was transporting the meteor 
to other countries. Similar phenomena presented themselves on 
the 14th of September. These observations, from which we may 
infer, that the aurora borealis belongs to the region of the higher 
clouds, seem to me to render probable an opinion generally pre- 
valent in all northern countries, which is, that the aurora borea- 
lis, when very vivid, is accompanied with a considerable noise, 
and in some cases with one of great violence. I am well aware 
how little reliance is to be placed on common opinion under cir- 
cumstances calculated to inspire terror, or when influenced by 
the frightful appearance of rapid and unexpected commotions ; 
but the assertions thus made, like all others, possess a degree of 
credibility ; and if it is unphilosophical to believe without proof, 
it is equally so to reject without examination. L( t a person 
apply himself for thirty years, to the study of what are called 
popular prejudices, and I doubt not his labours would be re- 
warded by many valuable discoveries. If any one will inquire 
without bias or prepossession into the reality of the sounds alleged 
to proceed from the aurora borealis, I am persuaded that he will 
not he t to adopt the common opinion, so striking is the coin- 
cidence of testimony on this subject. The distinguished natural 
philosopher Muschenbroek, who wrote about the middle of the 
last century, reports, thaUhis fact is generally affirmed by sail- 



286 Magnetism. 

ors employed in the whale fishery on the coast of Greenland. 
Gmelin, in his account of Siberia, expresses himself in still 
more decided language; after speaking of the great splendour of 
the aurora borealis, as presented in these countries, he adds ; 
" However beautiful this spectacle may be, I think it will be im- 
possible to contemplate it for the first time, without emotions of 
terror; so constantly is it accompanied, as I have been informed 
by several intelligent persons, with noises like those hissings and 
cracklings produced by very large fireworks. The hunters who 
go in search of the blue fox to the confines of the Frozen Ocean, 
are frequently surprised by the unexpected appearance of this 
meteor ; their dogs are frightened by it to such a degree that 
they cannot be kept from stopping and lying on the earth until 
the noise has ceased." There is a phrase belonging to the lan- 
guage of this country, used solely to express the terror which 
this phenomenon occasions. Gmelin adds, that there was a 
unanimous voice in support of what is here stated. I can affirm,, 
moreover, that among the inhabitants of the Shetland Islands, 
the testimony is no less full and complete, although they do not 
speak of so loud a noise ; a difference to be attributed undoubt- 
edly to the less northern situation of these islands. M. Edmon- 
ston, who, like myself, was unacquainted with tne passage just 
quoted from the work of Gmelin, described to me the noise oc- 
casioned by the aurora borealis in very similar terms, giving me 
to understand that he had very frequently heard it himself; he 
thought it most like the noise proceeding from a large fire. I did 
not have an opportunity of observing it during the appearance of 
the meteor when I was at Unst, as the sea then roared with great 
violence on the side of the island where I was. In fine, the inhab- 
itants spoke only of having heard the noise of the meteor, when 
the phosphoric jets are very numerous, and when they cross and 
intermingle with the greatest activity. For the truth of what is 
here alleged we may appeal w r ith confidence to the whole popu- 
lation of the Shetland Islands ; hardly a person is to be found who 
will deny having heard this noise ; we do not however depend on 
assertion merely ; it is described in the same manner by differ- 
ent persons, without their once imagining that there can be any 
doubt about it. The phenomenon seems to be much more bril- 
liant a few degrees nearer the pole. M. Edmonston, in an ac- 
count of the appearance of a large aurora borealis which he 



Aurora Borealis. 287 

observed at Unst on the 1st of November, 1818, has afforded me 
a striking example of this difference. " I am now in company," 
says he, " with two credible persons who on a voyage from Lon- 
don to the Shetland Islands, were driven by winds to the latitude 
of 63|°, near the northernmost extremity of the island. While 
they were in this latitude an aurora borealis appeared ; the noise 
with which it was accompanied was such that the sailors were 
afraid to remain on deck ; and it sent forth so strong a light, 
that we were able to observe the compass by it." It seems pro- 
bable after this mass of testimony, that the meteor sometimes de- 
scends so low as to allow us to hear the noise proceeding from it. 
It has even been affirmed by Bergmann, that persons travelling 
over the Norwegian Alps have been enveloped in it, and have 
perceived a strong smell of sulphur, supposed to proceed from it. 
238. Having thus collected the several particulars belonging 
to the aurora borealis, in doing which, I have endeavoured to 
exclude every thing of a hypothetical nature, we may consider 
this meteor as consisting of real clouds, proceeding usually 
from the north, and composed of some very light substances, or 
at least of some substance so finely pulverized as to be capable 
of floating a long time in the atmosphere, endued with the pro- 
perty of occasionally becoming luminous ; and especially (which 
is very important) sensible to terrestrial magnetism, and sponta- 
neously arranging themselves in columns which turn towards the 
earth, as real magnetic needles would do. But of all terrestrial sub- 
stances only the metals, so far as we know, are in any consider- 
able degree susceptible of magnetism. It is then probable, that 
the columns of the meteor are at least in a great measure com- 
posed of metallic particles reduced to powder of extreme fine- 
ness. But this conclusion leads also to another ; we know that 
all known metals are excellent conductors of electricity. Now 
the different strata of which the atmosphere is composed are 
usually charged with very unequal quantities of electricity; 
for if, when the atmosphere is most serene, we raise a paper kite 
with a metallic string, we may observe at the end of the string 
signs of electricity, ordinarily of the vitreous kind ; and if, on 
the other hand, having ascended in a balloon, we let fall below the 
car a wire whose inferior extremity shall reach the lower strata 
of the atmosphere, we shall find, as has been observed by M. 
Guy-Lussac and myself, that the superior end of the wire gives 



288 Magnetism. 

indications of resinous electricity. Accordingly, if columns con- 
sisting in part of metallic substances, are suspended in nearly a 
vertical position i' ; t k atmosphere, like the columns of the au- 
rora borealis when they float over regions adjacent to the pole, the 
electricity of the atmosperic strata at the summit and base of the 
columns will find in them so many conductors more or less per- 
fect ; and if this tendency of electricity to diffuse itself uniformly 
is sufficient to overcome the resistance arising from the imperfect 
conducting power of the columns, it will flow along these col- 
umns, illuminating its path, as is often observed in conductors 
which are not continuous. When this passage takes place in the 
higher regions of the atmosphere where the air, on account of its 
rarity, offers very little resistance, the electricity will flow on 
silently with all those variations of light which we observe in 
exhausted tubes. But if it extends itself to the inferior strata, it 
must necessarily occasion such hissing and crackling noises, as are 
found to accompany the aurora borealis, when it descends near 
the surface of the earth. In fine, as the meteor is visible only 
by means of this accidental circumstance, there is reason to be- 
lieve that it may exist in the air and exert an influence over the 
magnetic needle without being perceived ; it is also very possible 
that it may be bright in some places and obscure in others ; while 
under certain circumstances the disturbance of the electric equi- 
librium being sudden and general, the whole meteoric colonnade 
may be instantaneously illuminated. These phenomena must be 
less striking as the meteor advances over the more southern 
countries, not only because it has then extended itself more 
widely, bi t especiall because the conducting columns, always 
conforming to the direction of the magnetic needle, will become 
more and more horizontal, and will have their two extremities in 
atmospheric strata less distant, and therefore less unequal with 
respect to the quantities of electricity with which they are charg- 
ed ; a greater humidity also which prevails in the lower lati- 
tudes is favourable to a frequent discharge. 

All these results, agreeing so exactly with what we have col- 
lected from actual observation, it will be seen, depend solely on 
the idea, that the columns of which the aurora borealis is consti- 
tuted, are partly, at least, of a metallic nature. This agreement 
with known phenomena considerably increases the probability 
of the supposition to which we were previously led by the mag- 



Periodical Variations of the Needle. 289 

netism of the meteoric columns; the mutual connection and inti- 
mate dependance, thus easily established between phenomena so 
numerous and, at first view, so remote, gives an air of reality to 
the whole, seldom to be meet with in physical theories which 
have not the basis of established fact. 

239. But, independently of the luminous jets which may thus 
be produced by the simple passage of electricity along the me- 
tallic columns, a passage which in virtue of a property lately 
discovered, might of itself be sufficient to magnetize these col- 252. 
umns ; we can hardly help considering the phenomena in question 

as proceeding from an actual combustion in the phosphoric clouds, 
which, detaching themselves in some cases from the burning me- 
teor, as affirmed by many observers, and as I have myself seen, 
transport with them the principle of their phosphorescence, and 
emit at intervals jets of light resembling rockets, which leave after 
them a whitish train. We must then regard it as at least a very 
probable supposition, that the aurora borealis is composed of sub- 
stances, capable occasionally of inflammation, either of a spon- 
taneous kind, or in consequence of a discharge of electricity 
from the clouds which contain it ; a very powerful mode of 
combination, of which we have frequent instances in our labo- 
ratories. 

240. Such are the physical conditions on which the aurora 
borealis seems to depend, and which are deduced directly from 
the phenomena presented to us. Whence then is derived the 
matter which constitutes it ? To this question we can as yet give 
only a doubtful answer ; but if a skilful observer would, for sev- 
eral winters, carefully study every circumstance belonging to 
these phenomena as they present themselves in the northern 
regions, with all the helps that the sciences can furnish, something 
decisive would probably be learned. 

241. All the periodical or accidental variations which the 
needle undergoes may be measured with extreme accuracy by 
an apparatus invented by Coulomb and represented in figure 1 1 2, 
It is a box of wood or copper, glazed on the upper side, and 
having at each end a vertical microscope, provided with cross 
hairs, and capable of being moved along a graduated copper 
arc attached to the box. The needle, suspended edge-wise by 
untwisted silk threads, and rendered horizontal by the stiffness 
of the cap which supports it, has such an extent that its extremi- 

E. $ M. 37 



290 Magnetism* 



©• 



ties pass under the microscopes, and are thus accurately observ- 
ed. An extremely fine line engraved on each of them serves as 
a mark. A still better contrivance was invented by Gambey, 
which consists of the intersection of two cross wires stretched 
over a copper ring, each end of the needle being provided 
with this appendage. Having placed the whole apparatus on a 
perfectly stable support of stone, we first turn the box so that its 
longer side may coincide with the magnetic meridian, and the nee- 
dle direct itself very nearly to the middle point of the graduated 
arc. This condition being fulfilled, we fix the apparatus perma- 
nently on its support; and when the needle has become sta- 
tionary we move the microscopes gently by a finger-screw, until 
the point of intersection of their cross hairs exactly coincides 
with the mark or similar intersection on the needle ; and we 
note the time of observation. After an hour we visit the appara- 
tus again ; and if the needle has changed its position, we observe 
it and bring again the intersection of the cross hairs of the micro- 
scopes to the marks on the needle; and this change of place of 
the microscopes measured on the graduated arc that supports 
them, will show how much the needle has altered its position in 
any given time. 

This same apparatus may also be employed in measuring the 
variations of intensity of the magnetic forces. We have only to 
withdraw the needle from its direction of equilibrium, by pre- 
senting to it at a distance for a very short time a piece of soft 
iron, and then, removing the iron, to observe with a very accu- 
rate seconds-watch the time which it employs in making any num- 
ber of oscillations. An ingenious Norwegian observer, M. Han- 
steen, has found in this way that the intensity, like the declination, 
has its variations both annual and diurnal. It generally decreases 
from morning until about eleven o'clock, and then increases 
until four o'clock in the afternoon in winter, and six or eigrht in 
summer. Its minimum takes place in January, and its maximum 
in July. Without doubt the inclination is in like manner variable; 
and therefore, agreeably to what has been already remarked, we 
must take its changes into consideration, and also those of the tem- 
perature, in order to make a true estimate of the intensity as de- 
duced from the number of oscillations performed in a given time. 
I am unable to say whether M. Hansteen attended to these in- 
dispensable, corrections. 



Periodical Variations of the Needle. 291 

242. I will here make known a method by which the diurnal 
variations may be increased almost indefinitely. It consists in 
placing laterally, at some distance from the moveable needle, 
another needle either moveable or fixed, but of such a length, 
that the action of one of its poles shall greatly predominate. The 
reciprocal action of the two needles, arising chiefly from the 
attraction or repulsion between the contiguous poles, is combin- 
ed with the directive terrestrial force, which tends to bring them 
to the magnetic meridian ; and the position in which each is 
fixed, is determined by the resultant of these three kinds of 
forces. Now when the direction of the terrestrial component 
varies, this resuhant will also change its direction ; and it ap 
pears from calculation, that its change must in some cases be 
greater than that of the terrestrial force itself. We have, there- 
fore, only to make this favourable arrangement ; and if we are 
careful to use needles of very hard steel, highly tempered and 
incapable of acquiring a very high degree of magnetism, and 
consequently fitted to be more constant, I doubt not that we 
shall obtain an apparatus whose extreme sensibility will make 
known to us many curious phenomena, which the smallness of 
the diurnal variations has hitherto prevented our observing.! 

243. It must be considered as important for the future pro- 
gress of physics to determine with accuracy, the actual intensity 
of terrestrial magnetism, as it has been to determine the absolute 
pressure of the atmosphere, and the temperature of different 
climates. If the same observations are continued for several 
ages, it will be known whether there is any variation in the 
intensity of the magnetic forces, analogous to that which is found 
to exist with respect to its direction. 

The first method which suggests itself is to observe the decli- 
nation, the inclination, and the intensity, by means of three 
needles appropriated solely to these objects, and carefully pre- 
served for the purpose from age to age. As they may lose 
some portion of their magnetism during long intervals, they may 
be restored to the same degree of magnetic power, by being 
subjected anew to the process of magnetizing with the aid of very 
strong bars, combined according to the method of double touch. 
Indeed, if we apply this process, the needles will by the influ- 



t See note on the diurnal variations, 



292 Magnetism* 

ence of the extreme bars be immediately charged with a degree 
of magnetism much higher than they can retain when left to 
themselves. So that if their internal constitution undergoes no 
change, the degree of magnetic power with which they are sat- 
urated would remain constant, and of course the variations of di- 
rective force would afterwards depend solely on the changes that 
take place in the magnetism of the globe. We can render this 
method much more accurate by thus preserving a number of 
needles well proved ; but we must always be certain that they 
have remained untouched. We may dispense with this condi- 
tion, however, if we can discover any means of making two 
needles exactly similar at all times. For this purpose, we must 
not think of employing steel, which being a compound of carbon 
and iron, is necessarily variable in its proportions. But we may 
very successfully employ cylindrical needles made of a com- 
pound of wax and deutoxide of iron in known proportions ; for 
this deutoxide of which natural magnets are in a great measure 
composed, is very susceptible of magnetism, and very little liable 
to change its component parts. It will be sufficient therefore 
to make similar magnetic needles at dilferent epochs, and to 
magnetize them to saturation, and then to observe the effects 
produced on them by the terrestrial forces. 



Practical Instructions as to the Method of observing the Elements of 
Terrestrial Magnetism* 



244. Magnetic observations being one of the most import- 
ant objects that can engage the attention of travellers, I have 
thought it would be useful to subjoin a few practical instructions 
respecting the processes to be employed in making such obser- 
v t'ons with accuracy. To begin with the most simple case, I 
will suppose that the observations are to be made on land ; I will 
then describe the additional precautions necessary on board of 
vessels, liable always to be more or less agitated, and which may 
themselves exert a considerable disturbing force on the needle 
in consequence of the iron used in and about them. The first ele- 
ment to be determined is the declination, that is, the angle com- 



Practical Instructions. 293 

prehended between the magnetic needle and the plane of the 
astronomical meridian. The instruments destined for this pur- 
pose are called declination or azimuth compasses. Among the 
different forms which have been used, I give the preference to 
that invented by M. CasMni, to which our ingenious artist M. 
Gambey has added an improvement, that gives it a decided 
superiority over all others. It is represented in figure 123. 

This compass is composed principally of a long magnetic 
needle of a rectangular form, suspended edgewise in a horizontal 
position b^ an assemblage of flat silk threads without any sensi- 
ble torsion, and surrounded by a horizontal graduated circle, 
IlOF, which enables us to measure the extent of its motions. 
The point of suspension is at C in a cross bar of copper, sup- 
ported by two columns of the same metal ; and these columns 
are inserted at their bases into a plate also of copper, which 
rests on a pivot in the centre of a circle ; so that the whole ap- 
paratus admits of being turned about this centre, like a common 
surveying instrument. A branch of copper B, attached to one 
of the columns, carries a vernier V over the graduated circle 
£0, that is employed in measuring this motion. Moreover to 
guard the apparatus from the agitation of the air, it is completely 
enclosed in a glazed box of wood or copper, which rests upon 
the same metallic supports. Having assured ourselves of the 
perfect mobility of the needle, it only remains to determine the 
point of the horizon to which it directs itself. For this purpose, 
by means of the transverse axis AA\ we attach to the summit of 
the two columns a telescope LL, which is moveable in a vertical 
plane, the axis AA' being horizontal. To give the axis this posi- 
tion, we first make the graduated circle EOF itself perfectly 
horizontal by means of the adjusting screws v, i>, v, and spirit 
levels placed upon its surface. We then suspend a spirit level 
to the axis AA', by means of two hooks ; and if it is not already 
horizontal, we make it so by the aid of a little apparatus of 
movable pieces attached to the columns and admitting of a verti- 
cal as well as a horizontal motion in one of the ends A of the 
axis. Now the telescope LL contains in its interior two very fine 
hairs or wires, situated in the focus of the eye-glass, whose point 
of intersection serves to fix the precise direction of the visual 
ray. But these wires are so placed by the instrument-maker, 
that the visual ray which passes through their point of inter- 



294 Magnetism. 

section is exactly perpendicular to the axis AA\ and passes 
through its middle point. Besides, the telescope has not a sim- 
ple spherical lens for an object-glsss ; but two such lenses placed 
one over the other, very unequal both as to curvature and dimen- 
sions ; one, occupying the larger part of the tube, throws a dis- 
tinct image of distant objects on the wires ; the other, which is 
much smaller, is so formed that when combined with the larger, 
it throws on the same wires the image of very near objects. 
Moreover, the direction of the visual ray, which passes through 
the intersection of the wires, is regulated by the two lenses 
in the same manner. Accordingly, if we would see only very 
near objects with the telescope, we have merely to cover all 
that portion of the larger glass for which we have no use, by at- 
taching to the end of the instrument an opaque cover having a cir- 
cular opening at the centre, as represented in figure 124; and, on 
the contrary, if we would look at distant objects we substitute an- 
other cover, opaque at the centre and open toward the circumfer- 
ence, as shown in figure 1 25. This being well understood, we can 
determine the direction of the magnetic needle in the following 
manner ; we first turn the box until the needle attains a free 
and unobstructed position ; and when it is stationary we direct 
the microscopic part of the tube LL successively towards the 
two ends of the needle, where are attached the cross wires 
which serve for signals, like those on the needle employed for 
the diurnal variations. It seldom happens, that the point of in- 
tersection of the wires is, on the first trial, in a line with the 
intersection of the wires of the telescope ; but as we can move 
the axis of the telescope in a horizontal direction, and also turn 
it by means of the arm B attached to the columns, it is always 
very possible to bring the intersection of the wires of the teles- 
cope to coincide with the image of the signal carried by the 
needle ; and it is moreover necessary, that this coincidence 
should be effected at each end of the needle. When this condi- 
tion is fulfilled, the optic axis of the telescope, that is, the 
visual ray which passes through the intersection of the wires, 
will evidently be in the same vertical plane with the line drawn 
through the two signals, affixed to the extremities of the needle. 
This plane will then be that of the magnetic meridian, if the 
line above mentioned coincide with the magnetic axis of the needle. 
Let us suppose for the present that this is the case. We have, 



Practical Instructions. 295 

therefore only to take from the end of the telescope the cover by 
which it was fitted for near objects, and to substitute instead of 
it that which answers to the small lens, in ord?r that we may dis- 
tinguish distant objects ; then, directing the telescope to some 
point near the horizon, which is directly in the line of intersec- 
tion of the interior wires, we shall have the position of the mag- 
netic meridian ; and thus we may discover the declination of 
the needle by measuring, at our leisure, the angle comprehended 
between this line and the geographical meridian of the place. 
This problem belongs to astronomy. But it is not by any means 
certain that the line drawn through the two signal points of the 
needle is its magnetic axis ; here then is an occasion for apply- 
ing the method of correction already explained. The process 
in this case is very easy ; for the needle has for its cap a hollow 
ring to which is accurately fitted a copper cylinder that encloses 
it. In order to reverse it, therefore, it is sufficient to turn it up- 
side down by shifting the cylinder ; after which we observe 
anew the direction. If we obtain the same point in the horizon 
as before, we have no correction to make ; but if the second 
direction differs from the first, as is most generally the case, we 
must refer them both to the geographical meridian, and take 
the mean of the angles thus observed. This will be the true 
declination. 

245. This reference to the meridian can be very accurately 
made with the same instrument. For, when we have found the 
point of the horizon, to which the axis of the telescope is direct- 
ed, we observe the number of degrees, &c, in the horizontal 
circular division to which the vernier of the arm B corresponds. 
This being done, without touching this circle again or deranging 
it at all, give free motion to the box and columns, without now- 
regarding the needle, and turn the arm B, until the telescope is 
directed towards some known star then situated near the horizon. 
Observe, by means of a good watch, the precise moment when 
this star is directly behind the point of intersection of the wires, 
and we can hence deduce by calculation the angle comprehend- 
ed between the geographical meridian and the vertical plane in 
which the star is situated at this instant. But having noted the 
point of the graduated circle to which the vernier of the arm B 
corresponds, we shall know the angle which this same plane 
makes with the magnetic meridian, in which the telescope was 



296 Magnetism. 

at first directed ; we shall thus obtain the angle formed by this 
meridian and the geographic meridian. 

246. As we have seen that, in the same place, the magnetic 
needle undergoes slight periodical variations, it is necessary in 
order to obtain an accurate estimate of the declination, to repeat 
these observations at such days and hours that the variations 
may balance each other by being in direct opposition ; we must 
also be on our guard against those circumstances under which 
the needle, employed for the diurnal variations, has indicated the 
existence of disturbing causes. In general, if we aim at great 
accuracy, it is indispensably necessary to note the day and hour 
of the observations. As to the diurnal variations themselves, 
which it is an object of no less interest to observe in places 
remote from each other, no means more perfect can be devised 
for measuring them than the apparatus of Coulomb, already de- 
scribed. 

The method just explained for finding the absolute declina- 
tion, is essentially the same with that used for observing the de- 
clination of the magnetic needle at sea ; but in order to render it 
fully applicable to this purpose, certain modifications are necessa- 
ry. We must, in the first place, dispense with the telescope, which 
it would be almost impossible to use on account of the motion of the 
vessel, and substitute for it simple threads, stretched vertically 
over plates of copper adapted to the circumference of a box 
which is capable of turning freely in the interior part of the ap- 
paratus. The plates have slits cut in them against the threads, 
and are termed sights. They are so placed that the two threads, 
determining the direction of the visual ray, fall at the two extremi- 
ties of a diameter of the circular division. But this division is 
not in the present case traced on a fixed circle ; but on a light 
disc of pasteboard or horn, which the needle itself carries and 
directs, its northern point being placed on the division 0°. Be- 
sides, as it is not in our power to place the instrument on a fixed 
plane, we are obliged to suspend it so that it may partake as lit- 
tle as possible of the motion of the vessel, and that it may always 
tend by its own gravity to the horizontal position necessary for 
observations. For this purpose we employ gimbols. a method 
of suspension represented in figure 126. In the first place, the 
instrument is attached to the axis a «, which turns freely on two 
opposite points of the circle of copper ; and this circle in its turp. 



Practical Instructions, 237 

"i^ in like manner, suspended to another axis b b perpendicular to 
the former ; so that if we incline the exterior supports of the 
instrument in any manner whatever, provided we do not exceed 
certain limits, the box, suspended on the first axis a #, will remain 
upright in all positions of the vessel, and indeed its own weight, 
will always restore it more or less readily to this position ; so 
that there is the least possible disturbance of the needle, espe- 
cially when it is so adjusted that its centre fails at the point of 
intersection of the two axes of suspension. When the azimuth 
of an object is to be taken with this instrument, we turn the box 
containing the needle until the threads of the sights are directed 
towards it.; and as the needle, on account of the directive force, 
does not partake of this motion, the diameter of the circular 
division, answering to the threads, indicates the angle compre- 
hended between the direction of the object and that of the 
needle. In order to facilitate this operation, the artist traces on 
the interior of the box two fixed lines on a level with the gradu- 
ated disc which the needle carries. We may immediately deter- 
mine the numbers of the division to which these marks corres- 
pond, when the diameter answering to the threads coincides 
with the direction of the needle ; and then the numbers against 
which it falls when the box is turned a certain angle will meas- 
ure the amount of the deviation. When the observation is made 
at sea, two observers are necessary; one directs the sights, while 
the other determines upon the box the mean place of the needle, 
which is continually agitated by the motion of the vessel. 

247. I cannot leave the subject of declination compasses with- 
out adverting to a very ingenious improvement made by Captain 
Kater, which adds greatly to the accuracy of such observations, 
while at the same time it facilitates them. It consists in placing 
the observer so that he can see, at the same time and with the 
same eye, a very fine thread which projects itself on the distant 
object whose bearing is to be taken, and also the point of the cir- 
cular division which answers to the direction of the visual ray 
coming from this object. We effect this double purpose, by 
causing the image of the distant object to reach the eye, by 
direct vision, and that of the circular division, by reflection from 
an inclined mirror. 

After this suggestion, in order to have a perfectly clear idea of 
the process, we need only a description of the instrument. It is 

E. & JSL 38 



208 Magnetism. 

represented in figure 127. It consists of a copper box whose diam- 
eter is about two and a half inches, and whose circular bottom 
suj oris a steel pivot with a fine point, on which is placed the 
centre of the magnetic needle, the cap used for this purpose 
being of agate that the motion may be more free. This needle, 
like those of the mariner's compass, carries a light circle 
of pasteboard or horn, with a graduated circumference, zero 
coinciding with the north point. The whole is covered with 
very transparent glass which preserves the needle from the agi- 
tations of the air. The apparatus substituted for sights is com- 
posed of two pieces A and B] the first, A, is a plate of copper 
fixed perpendicularly to the plane of the box, having a slit cut 
in it, through the middle of which is stretched a very fine thread, 
that must during the observation remain vertical and perpendicu- 
lar to the plane of the circular divisions. This condition may be 
fulfilled. by attaching to it a small weight, and levelling the box 
until it strikes against the fixed point F, marked at the foot of the 
plate. Opposite to this plate of copper is the piece B, where 
the eye is applied. It chiefly consists, 1. of a small hole 7*, 
through which we are to look at the direction of the thread F, and 
the object selected for the point of sight. 2. Of a small piece 
of a hemispherical lens, doubly convex, designated by C, which 
by magnifying enables us to see the degrees of the circular divis- 
ion, the image of which is reflected by a small silver mirror M, 
As the pupil has a sensible diameter, we shall be able to see in 
these two ways at the same time. Then the vertical thread ap- 
pears like a slender mark on the reflected image of the divisions, 
which are diametrically opposite to it ; and this superposition 
determines with no less facility than exactness the direction of 
the visual ray. For example, the instrument being horizontal, if 
we turn it until the thread is projected on 180°, the line of vision 
will exactly coincide with the direclion of the needle, and the 
declination of objects situated in that direction will be nothing. 
But if we turn the box horizontally through a certain angle by 
which means the visual ray is directed towards other objects, 
the needle which remains constantly directed towards the same 
point of space, will preserve unchanged the circular division, and 
the sight thread will be projected on some other point of the 
divisions, and we may thus measure the angle passed over-. 



Practical Instructions* 299 

-248. We come now to speak of the manner of observing the 
magnetic inclination. The instrument to be employed, called the 
dipping needle, has been so particularly described that it is un- 
necessary to advert to its construction here. I will only remark, 
that it contains the same apparatus for levelling as the declina- 
tion needle, and that it is in like manner placed upon some solid 
support when used on land. But at sea it is suffered to hang 
freely by a ring attached near iis upper surface, which makes a 
part of the suspension by gimbols. In order to observe the incli- 
nation in the two cases, we must first bring to the magnetic me- 
ridian the vertical plane which contains the needle; and the 
angle in question is determined by the vertical circle itself, in 
the centre of which the needle is suspended. This may be done 
in three ways. 1. By reducing this plane to the direction of 
the magnetic meridian previously determined. 2. By first seek- 
ing the direction of the azimuth, in which the needle is exactly 
vertical, and then turning this plane 90°. 3. By turning and 
preserving the limb in the direction of the azimuth, in which the 
inclination of the needle is the least possible. Although this 
last method is less precise than the two others, it still gives re- 
sults of very considerable accuracy, since for a space of some 
degrees on each side of the magnetic meridian the inclination is 
nearly the same as in the plane itself. Indeed, it is the only 
method that can be employed at sea, because the continual agi- 
tation of the vessel does not allow us to establish any fixed rela- 
tion between two absolute and successive positions of the plane 
of the limb in space. 

The limb of the instrument being placed by one or the other of 
these methods, exactly or very nearly in the direction of the mag- 
netic meridian, let us call that face of the needle which is direct- 
ed towards the east, E, When in this position, we must care- 
fully note the point of the division at which the needle becomes 
stationary when on land ; or, which is equally accurate, and 
admits also of being applied at sea, we notice the extreme limits 
of the oscillations when these expend only through a small space. 
We make these observations at each end, and take the mean of 
the results, by which method we avoid the error arising from 
the excentricity of the needle, if it does not happen to be sus- 
pended exactly in the centre of the circular divisions. This 
supposes that the two points of zenith and nadir in the gradua- 



300 Magnetism* 

tion, are exactly in the same vertical. On land we may re- 
duce them to this position by making the instrument horizontal 
by means of the levels attached to the circular base. At sea 
this cannot be done, except by taking a mean of ten or twelve 
observations, in the course of which, on account of the motion of 
the vessel^ the zenith of the divisions will vibrate on each side of 
the true zenith. Having thus observed the inclination with the 
face E of the needle turned towards the east, we repeat our 
operations with the same face E turned towards the west ; tak- 
ing care to use all the precautions recommended in the first ex- 
periment. 

This turning of the instrument, as in the case of the declina- 
tion needle, serves to correct any error arising from the position 
of the magnetic axis of the needle, which differs but little for the 
most part from the geometrical axis, but does not always coin- 
cide with it. The precaution therefore should never be omitted. 

The mean of the four observations thus obtained would be the 
true inclination, if the needle were suspended exactly by its cen- 
tre of gravity. But however careful the artist may have been 
to effect this, he seldom succeeds in doing it with mathematical 
accuracy* Hence the excess of weight in one arm of the needle 
must increase or diminish the inclination. But this error may 
be corrected by the method already explained. For this pur- 
pose we remove the needle, and reverse its poles by magnetizing 
them anew with powerful magnets. We then repeat the four 
observations above described. We shall thus obtain a new 
value for the inclination, the error of which will be directly op- 
posite to that of the first set of observations, and if the needle is 
carefully made, by taking the mean between them, we shall 
obtain the true inclination. This operation of changing the poles 
should never be dispensed with except from absolute necessity* 

249. It now remains for us to speak of the intensity of mag- 
netic forces. It may be deduced from the oscillations of a hori- 
zontal needle, combined with the observed value of the inclina- 
tion. Let JV be the number of seconds employed by the needle 
in making a certain number of horizontal oscillations, in a place 
where the intensity of magnetic force is designated by i?, and 
where the zenith distance of the magnetic axis is Z ; if we denote 
by JV, jR 7 , Z', corresponding quantities for any other place, we 
shall have 



Practical Instructions* 301 

N 2 sin Z 



N 2 sin Z 

~ N' 2 sin Z r 



This method may be employed especially when we are not in 
the neighbourhood of either magnetic pole ; but to render it ex- 
act several precautions are necessary. In the first place, we 
must endeavour to suspend the needle in such a manner as to 
avoid entirely the influence of torsion. This may be done by 
attaching it to a collection of flat untwisted silk threads. The 
horizontal position of the needle is then effected by simply plac- 
ing it in a paper dish, the weight of which must be so small 
as to have no sensible effect. Of all the different forms the best 
for this kind of observations is a long thin parallelopiped. Care 
must be taken to suspend the needle with its broad surface hori- 
zontal, and not edgewise, in order to avoid as much as possible 
the resistance which the air opposes to its oscillations. 

250. In the preceding remarks we have considered terrestrial 
magnetism, as the only force exerted on the needles whose mo- 
tions are to be observed. Indeed, we may reduce all experi- 
ments to this simple case by taking care to remove every mag- 
netic substance, and to have about our persons, at the time of 
observing, no key or other ferruginous instrument. But it is out 
of our power to do this when at sea, not only on account of 
the great quantity of iron used in the construction of vessels, but 
also from the circumstance of the arms, cannon, and iron utensils 
of every sort which cannot be dispensed with. All these masses 
united must exert over the compass needle an influence which 
is combined with that of the terrestrial globe, and must conse- 
quently modify its direction and its motions. 

In order to analyse the effects produced by this action, we 
must first remark, that it may be referred to three distinct causes. 
1. It may proceed from a permanent magnetic power imparted to 
ferruginous masses by the processes necessary to prepare them 
for use ; 2. or it may arise from these masses being accidentally 
thrown into a magnetic state by the influence of terrestrial mag- 
netism ; 3. lastly, it may be referred to a like magnetic state deter- 
mined by the influence of the compass needle itself on the fer- 
ruginous masses by which it is surrounded. These three causes 
of deviation exist together or apart, conspire or oppose each 
other. 



302 Magnetism. 

251. We can easily reduce the effect of the last cause so as 
to render it wholly insensible, by placing the compass in such a 
situation, that no considerable mass of iron shall be in its neigh- 
bourhood ; but we cannot proceed in a similar manner with 
respect to the two other causes. As their energy does not de- 
pend on the needle, we cannot discover their limits. Happily 
the different effects which they are capable of producing will 
enable us to distinguish them. 

Let us begin with an examination of the first kind of action, 
namely, that which proceeds from a durable magnetic state be- 
longing to ferruginous masses. In whatever manner these masses 
may be distributed in the vessel, and whatever may be the na- 
ture and intensity of free magnetism in each of them, if they are 
sufficiently removed from the needle, as the first condition sup- 
poses, we may always combine their action into two resultants, 
one boreal, the other austral, of equal intensities, and whose 
direction relative to the axis of the vessel will depend on the 
distribution of magnetism in these masses, and also on their rela- 
tion to the compass and to each other. This intensity and this 
relative direction will remain constant, whatever be the direction 
of the axis of the vessel, whether it turns to the east, the west, 
the north, or the south. The resultant with which it acts will 
only turn with it about the vertical describing the same number 
of degrees. But it is not so with the directive terrestrial force. 
This, always acting in the same direction, since it does not de- 
pend on the motion of the vessel, will always tend to restore the 
needle to its proper direction, that is, to the magnetic meridian 
of the place. The needle will then be attracted at one and the 
same time by two directive forces of constant intensities, but of 
which one only has a fixed direction, the other turning contin- 
ually and at the same rate with the vessel. With the knowledge 
of what is here stated we shall be able to assign numerically the 
law of the deviations to which the needle is subjected by these 
combined forces. 

The verification of this law is \evy easy. We have only to 
avail ourselves of some moment when the vessel is at anchor in 
a safe and quiet harbor; then choosing some distant object in the 
horizon as a signal, we direct the axis of the vessel towards it, 
and measure the angle formed by this axis with the direction of 
the magnetic needle. When this is done we turn the vessel a 



Practical Instructions, 303 

certain number of degrees to be measured by reference to some 
fixed signal, and again measure the angle comprehended between 
the axis of the vessel and the direction of the needle. We repeat 
the same observations till we have gone through the whole cir- 
cuit of the horizon, and the vessel returns to its first position. 
At the same time an observer is stationed at the same signal, 
with another compass, carefully compared with the one on board 
the vessel, to determine the angle which the line of the needle's 
direction makes with the line drawn from the signal to the ves- 
sel. By transferring this angle to the vessel, we have the quan- 
tity by which the line of sight actually differs from the magnetic 
meridian, as determined by the sole action of the terrestrial mag- 
net, whence we can deduce the direction and amount of the local 
deviation experienced by the magnetic needle at sea, in each 
position of the vessel. When we apply to observations of this 
kind, the formula theoretically deduced from the hypothesis of a 
constant disturbing force; we find that it answers sufficiently 
well for places at a moderate distance from the magnetic equa- 
tor ; but the error increases as we proceed to higher magnetic 
latitudes. 

Another striking proof, that the oscillations thus observed 
are not simply the effect of a constant magnetic action be- 
longing to the ferruginous matter in the vessel, is, that in the 
same vessel, laden in the same manner, the same needles undergo 
variations whose amount and laws become more complicated 
as we ascend to higher latitudes. If the deviations resulted 
wholly from a magnetic action within the vessel, and constant in 
all latitudes, the effect would increase indeed as we approach the 
magnetic terrestrial pole, since, the resultant of the terrestrial 
forces then approaching to a vertical direction, the horizontal com- 
ponent, derived from it and which is the directive force of the 
compass needle, would necessarily become more and more fee- 
ble, and this is one of the causes which render observations for 
the declination in high latitudes so uncertain, the slightest foreign 
magnetic force that acts on the needle being then sufficient to 
cause great errors. But this diminution of the magnetic power 
in the horizontal resultant, can also be calculated from the ob- 
served inclination ; and thus we can take account of it. Yet we 
find it far from being sufficient to account for the changes which 
take place in the absolute quantity of the deviations, and the 



304 Magnetism. 

manner of their varying according lo the different positions of 
the vessel. 

We infer then, that the phenomenon depends, at least in part, 
on the instantaneous developement of magnetism, produced in the 
ferruginous matter of the vessel by the influence of the terres- 
trial globe. And from the difficulty attending our inquiries into 
the manner in which electricity and magnetism are distributed 
in any body, however simple its form, even though it were only 
a portion of a cylinder or a cone, it will be seen how complica- 
ted the problem must be when the subject of investigation con- 
sists of irregular masses, distributed as to the magnetic com- 
pass without order or law. Jt is very evident, that calcula- 
tion is wholly out of the question ; so that there remains but 
one method of determining the law of the deviations, and that is, 
by comparing experimentally the directions of the needle at sea 
and on land for different positions of the vessel, as we have ex- 
plained above. This was done by the English officers in their 
first expedition to the polar regions. Now we have thus found,, 
that not only the exact amount of the deviations, but even the 
law by which they are governed, changes in different places ; 
and this indeed ought to be the case according to theory ; for, 
since there is a motion of the vessel about the vertical, it presents 
the ferruginous masses to the influence of terrestrial magnetism in 
different directions, and thus occasions magnetic states of differ- 
ent degrees of intensity. It is only on the magnetic equator 
itself, and for a short distance from it, that the laws of this 
change are capable of becoming more simple; since the direc- 
tion of the terrestrial forces being then horizontal, the develope- 
ment of magnetism is of the same intensity, but of a contrary 
kind in all positions of the vessel, which are 180° distant from 
each other. Whence it follows, that by taking the half sum of 
the deviations in each of these two opposite points, the errors 
will mutually destroy each other, and the mean of the whole will 
be the true declination; this result agrees with the observations 
of Captain Flinders, who first proposed and applied the method 
of correction which consists in inverting the needle. 






ELECTRO-DYNAMICS. 



Disturbance of the Magnetic Needle by the Electric Current. 

252. M. OersteDj of the Academy of Copenhagen, discovered 
in 1819 a phenomenon altogether remarkable, namely, the action 
of the voltaic current upon the magnetic needle. This discovery 
has led to a great number of valuable researches in France and 
other parts of Europe. 

We shall state the principal facts known at the present time 
upon this new branch of natural philosophy. 

The discovery of M. Oersted may be described in a few words. 

If we bring near to a magnetic needle a portion of a conduct' 
ing wire that unites the two extremities of a voltaic apparatus in 
action, we shall sec that this needle is turned from its direction ; 
and it is evidently the current that produces the deflection, since, 
if we interrupt it, the needle returns immediately to its former 
position.* 

If the power of the voltaic apparatus is enfeebled, the deviation 
of the needle becomes less. 

The common electrometer indicates the intensity of the electric 
tension. There was wanting an instrument, that should make 
known the presence of the electric current in a conductor or 
voltaic apparatus, and which should indicate its direction and 
energy. Such an instrument we now possess in the magnetic 
needle. 

When the facts discovered by Oersted were made known in 
France, M. Ampere analyzed them, and showed that they are 
reduced to the two following. 

* The needle is also deflected by common electricity, and that 
which is drawn from a cloud by a lightning rod. — M. Colladon, 
E. fy M. 39 



306 Electro-Dynamics. 

First fact. Directing Action. 

253. Suppose that a voltaic apparatus is placed horizontally 
and nearly in the direction of the magnetic meridian, and that a 
portion of the conducting wire is arranged in the same direction ; 
suppose, moreover, that a magnetic needle is placed above or 
below the conducting wire, it will be deflected in a manner that 
may be easily determined by the following rule. Let a person 
imagine himself placed in the direction of the current with his 
face toward, the needle, in such a way, that the direction of the 
current shall be from his feet to his head ; the austral pole of the 
needle will always be carried to the left by the action of the elec- 
tric current. 

It may be ascertained by the same instrument, that the current 
exists in the voltaic apparatus, and that it takes place from the 
resinous to the vitreous extremity, that is, in a direction the re- 
verse of that in the conducting wire ; a necessary result from the 
circumstance, that the conducting wire forms, with the voltaic 
apparatus, what is called a closed circuit. 

The electric current tends to put the needle in a position 
perpendicular to its proper direction ; but the action of the eartji 
prevents our obtaining this result ; so that the needle takes a 
position oblique to the conducting wire. If we destroy the influ- 
ence of the earth, as M. Ampere has done, by fixing a magnetic 
needle perpendicular to the axis of the dipping needle, the needle 
with respect to which the action of the earth is thus neutralized, 
will place itself so as to make a right angle with the electric 
current.* 



* There are three ways of rendering a magnetic needle astatic. 
The first is that already described in the text. The second, employed 
by M. Biot, consists in placing the two poles of a very powerful 
magnet in the magnetic meridian in such a manner that the austral 
pole -shall be to the north, and the boreal pole to the south ; and we 
vary the distance between this and the needle to be rendered astatic, 
until the action thus exerted, which is always opposite to that of the 
earth, shall be just equal to this last ; and this point is attained when 
the needle inclines to no one position rather than another. The 
third method, which is that of M. Ampere, is represented in figure 
128. We attach to a vertical copper wire ABC, movable upon the 



Disturbance of the Magnetic Needle. 307 

M. Ampere was led to the construction of this instrument by 
the consideration, that, when a magnetic needle can move only in 
a plane by turning about an axis perpendicular to this plane, it is 
always brought by the action of the earth into the situation, in 
which it makes the least possible angle with the direction of the 
dipping needle, which it would take if it were free, so that its 
austral pole makes the nearest possible approach to the austral 
pole of the dipping needle. It hence follows, that, if we would 
find the direction of the needle in any plane whatever, we must 
project the direction of the dipping needle upon this plane. The 
line of projection would be that in which the needle would take 
its position. Now it is evident, that, if the plane is perpendicular 
to the direction of the dipping needle, that of the needle in ques- 
tion making always a right angle with it, and being incapable of 
approaching to or receding from it, there is no tendency to any 
one position rather than another. The apparatus under considera- 
tion is represented in figure 129. The magnetic needle moves 
in the plane of a graduated circle. We give to this circle, and 
consequently to the needle, any position we choose, by means of 
the hinges a and a'. The strips of glass c, c, support the con- 
ducting wire of the voltaic apparatus. 

Second fact. Attractive and Repulsive Action, 

254. The second fact consists in this, that a conducting wire, 
and a magnet whose axis makes a right angle with the direction 
of the wire, attract each other, when the austral pole is to the left 
of the current which acts upon it; that is, when the position is 
such as the conducting wire and magnet tend to take, in virtue of 
their mutual action ; it being well understood, as M. Ampere has 
remarked, that it is necessary in order that this attraction may 
take place, that the straight line, which measures the shortest dis- 
tance between the wire and the axis of the magnet, should meet 
this axis between the two poles. This observation is so much the 
more important, as it explains why the attractive action becomes 

point C, two similar needles of steel, whose magnetism is of the same 
intensity, and whose poles are in opposite directions ; so that the earth, 
acting with equal and opposite forces upon the two needles, in all posi- 
tions which they are capable of taking, may be considered as having 
no influence upon them thus united. 



308 Electro-Dynamics. 

nothing opposite to the pole, and changes to repulsion when the 
straight line, measuring the shortest distance between the conduct- 
ing wire and the axis, meets this axis beyond the pole. On the 
other hand, a repulsion takes place when the austral pole is to 
the right ; that is, when the conducting wire and the magnet are 
maintained in a position opposite to that which they tend to as- 
sume ; provided always, that the straight line which measures the 
shortest distance falls between the two poles ; for, when it falls 
without, attraction takes place. 

The action between the conducting wire and the magnet is 
always reciprocal, in the cases above stated, as may be easily 
shown by bringing a magnet near to a movable conductor. 

255. We proceed now to explain, as methodically as the nature 
of the subject will allow, the facts hitherto made known on the 
subject of the attractions and repulsions of conducting wires. But 
we must observe, once for all, that these attractions and repulsions 
differ essentially from those heretofore spoken of, as belonging to 
electricity in a state of repose. In ordinary electrical attractions 
and repulsions, it will be recollected, that it is bodies possessing 
opposite electricities that attract each other, while those having 
the same electricity repel each other. The contrary takes place 
with regard to electrical currents. Attraction exists between 
those that move the same way, and repulsion between those that 
have opposite directions. Moreover, if two conducting wires are 
attracted to each other, they remain attached as long as the cur- 
rent continues, whereas in common electricity repulsion follows 
attraction more or less promptly. We may add further, that the 
effects of electrical currents take place in a vacuum as well as in 
the open air, while common electricity is dissipated when the 
atmosphere is removed. 

Attraction and Repulsion of Electrical Currents. 

256. Soon after the discovery of M. Oersted, it was found 
by M. Ampere, that two electrical currents attract each other 
when they are parallel and directed the same ivay, and that they 
repel each other when they are parallel and directed opposite 
ways. He discovered afterwards, that the same thing occurs, 
whatever be the angle of the two wires, acute, right, or obtuse ; 



Attraction and Repulsion of Electrical Currents. 309 

that is, there is always an attraction when the two currents pro- 
ceed in such a manner as to approach both toward, or to recede 
both from, the vertex of the angle formed by the two wires ; and Fig. 130. 
repulsion in the opposite case, in which, while one approaches, the 
other recedes from the same vertex. Fig. 131. 

Moreover, in order to give to the law above stated all the 
generality of which it is susceptible, it may be remarked, that 
where the conducting wires are not in the same plane, we may 
consider as the vertex of the angle formed by the wires the line 
which measures the shortest distance between them. 

If the two conducting wires are parallel, they are to be regard- 
ed as making an infinitely small angle with each other, the vertex 
of which is at an infinite distance. 

The repulsion in these cases is equal to the attraction, as may 
be shown by the following experiment. 

A conducting wire, bent back upon itself in such a manner that Fig- 132 - 
the two portions AB, BC, shall be separated from each other 
only by the silk with which they are covered, has no action upon 
another wire DE. 

The action of a rectilinear current is the same as that of a sinu-> 
ous current which deviates but little from a straight line ; as may 
be shown by substituting, in the preceding experiment, the sinuous 
wire for the rectilineal wire BC. Fig. 133. 

257. Two contiguous portions of the same rectilinear current 
may be considered as two currents forming with each other an 
angle of 180°, the vertex of which is at the point which separates 
them. It will hence be seen, that, the current of one of the por- 
tions, proceeding toward the vertex, and that of the other re- 
ceding from it, there ought to be a repulsion, as M. Ampere has 
found by the following experiment. Upon the dish AB CD, Fig. 134. 
separated by the insulating partition A C into two portions of the 
same size, and filled each with mercury, we place a copper wire 
covered with silk, the two branches q r, pn, being made to float 
upon the mercury parallel to the partition, while the uncovered 
extremities r s, nm, touch the surface of the mercury. Putting 
now the end of the vitreous wire in the cup E, and that of the 
resinous wire in the cup F, or the reverse, we establish two 
currents independent of each other, each of which has for a 
conductor a portion of the mercury and a portion that is solid. 
Whatever be the direction of the current, it will be seen, that the 



310 Electro-Dyna?nics. 

two wires qr, p n, move off parallel to the partition AC, in the 
direction opposite to that in which the instrument is in communi- 
cation with the voltaic apparatus, which indicates a repulsion for 
each wire between the portion of the current established in the 
mercury and its prolongation in the wire itself. The motion of 
the wire is more or less easily effected according to the direction 
of the current, since, in the one case, the action of the earth upon 
the horizontal portion qp, concurs with the repulsion in question, 
and, in the other, it is opposed to it, as we shall show more fully 
hereafter. 

258. If a current rectilinear and indefinite in both directions, 
act upon a movable current, placed perpendicularly to it, and 
having one of its extremities near it, the former will carry the 
latter parallel to its position, and the motion ivill take place in 
the direction of the indefinite current, when the movable current 
recedes from it, and in the opposite direction when the movable 
current approaches it. This will be rendered evident by a slight 
Fig. 135. inspection of figures 135, 136. For if we take, upon the current 
MN, from the point o, where it approaches nearest to the mov- 
able current, two equal distances o m, on, and from any point c of 
the movable current draw two oblique lines cm, c n, they will be 
equal, and will make equal angles with the directions of the two 
currents ; whence it follows, that the two small portions of the 
indefinite current, situated at the point m and n, act with the same 
intensity upon the small portion situated in c. But, of the two 
equal forces cp, cq, produced by these two small portions, the one 

256. is attractive and the other repulsive ; hence, by forming a paral- 
lelogram upon their directions, this parallelogram will have for its 
diagonal the line cR which is the resultant of the two actions, and 
evidently parallel to J)1JY. The resultants of the actions exerted 
upon the other points of the movable current being also parallel 

256. to MN, it will be the same with the total resultant. Now, from 
what we have just said respecting the mutual action of two cur- 
rents, it will be seen, that the action between m and c is repulsive, 
and that between n and c attractive. Whence it follows, that the 
movable current is carried in the direction c R, which is that of 
the indefinite current MN. . The reverse takes place in figure 
136. 

The action of these two currents is necessarily reciprocal, so 
that, if the conductor MN be the movable one, and if it could 



Attraction and Repulsion of Electrical Currents. 3 1 1 

slide along the straight line MN, it would proceed from n toward 
m in figure 135, and take the contrary direction in figure 136. 
This is not easily verified with a rectilinear current, but, by substi- 
tuting a circular current, we may readily obtain a similar result. 

This experiment is due to M. Savary. A circle of copper, Fig. 137. 
interrupted in A by a small piece of ivory CA, is immersed in an 
acid solution, contained in a metallic vessel that communicates 
with one of the extremities of the voltaic apparatus, the other 
extremity communicating with a small cup of mercury, in which 
terminates the point p that supports the circle, and which is united 
to this circle by the radius OC ; another radius OE helps to 
support the circle, but is of an insulating substance. It will be 
seen, that the instrument turns constantly in the direction AEC, 
whatever be the direction of the current which traverses it. Sup- 
pose, in the first place, that the current comes by the centre, it 
will reach the point C by the radius OC, follow the copper circle 
CEA, traverse the acid solution by the lines I m n,V in' n', &tc, 
perpendicular to the circle, and arrive at the metallic vessel. 
The whole current does not immediately pass into the acid solu- 
tion, on account of the obstacle which exists in the contact of two 
bodies, and on account of the feebleness of the conducting power 
of the liquid compared with the conducting power of the metal. 
Take a point I upon the circle. In the angle m I h there is a 
repulsion between the water and the circle ; but in the angle 
m I k an attraction takes place. The attraction and repulsion 
concur to give to the circle a motion in the direction k h or 
AEC, and to the water a motion in the opposite direction. This 
last motion is insensible on account of the liquid mass which is so 
considerable. If, on the other hand, the positive pole communi- 
cates with the external vessel, then the electrical current would 
reach the circle perpendicularly, according to the directions 
n ml, n' m' V> &tc, and would take the course AECO to ar- 
rive at the cup. Repulsion always taking place in the direction 
m h, ro' h'i the circle would turn in the same manner as before. 
It may be remarked, that, in the first case, AEC is opposite to the 
current in the movable conductor, and in the second case it is 
in the direction of the current. We can only change the direction 
of the rotation, by substituting for the above circle another circle, 
represented in figure 138, in which the insulating part AC is 
changed to the other side of the conductor OC. 



^ 



312 Electro-Dynamics. 

259. If the movable current traverse the indefinite current by 
extending itself equally on both sides, the direction of the mov- 
able current will be such as to approach it in the half BA, and to 

Fig. 139. recede from it in the half A C ; the first will be carried in the 
direction BE, opposite to that of the indefinite current, and the 
other half AC in the direction CF, which is that of the indefinite 
current. We have, therefore, a couple, composed of two forces 
BC, CF, which would no longer produce a motion of translation, 
but would cause the movable current BC to turn about the point 
A until it became parallel to MJV, and directed the same way. 
It may be remarked, that in this experiment, as in all those in 
which the movable conductor does not strike against the fixed 
conductor, the motion, on account of the velocity acquired, takes 
place beyond the position of equilibrium and then back again in 
a series of oscillations. 

Fig. 140. 260. Let us suppose now an indefinite current J\LV, and a 
current BA in a conducting wire, movable about one of its ex- 
tremities A, so that it can describe about the point A a circum- 
ference ; when the current comes from the circumference to the 
centre, that is, in the direction BA, and is in the situation marked 
BA in the figure, it will recede from MJST, and will consequently 
be carried in the direction of the current JUJV, which will bring 
it into the position B'A. It will then be parallel to MJV, and 
have the reverse direction ; consequently it will be repelled and 
take the position B" A. Then, as it approaches the current S\L\, 
it will be carried in a direction opposite to this last, and will take 
the position B'" A ; being now parallel to J\LY, and directed the 
same way, it will be attracted, and will take the position BA, 
whence the same motion of rotation will continue indefinitely. 

So long as the current MJV is near the point A, the action 
exerted by this movable current upon the wire, goes on diminish- 
ing from the position BA to the position B"A, since the distance 
between the two wires increases ; but, if the current MN were 
very far from A, the difference in the distance would become 
insensible, and then the calculus shows, that the action of MJV, 
in turning AB about A, is the same in all the positions which 
the movable conductor successively takes. The action may be 
rendered uniform, even for a small distance, by bending the fixed 
conductor in such a manner as to form a circle about A ; for it 
is then clear, that its action in turning AB about the point A 
becomes constant. 



Attraction and Repulsion of Electrical Currents. 313 

In this experiment we make use of a metallic vessel DEF, Fig. 141. 
about an inch and a half in depth, in the bottom of which, at the 
centre, is inserted a small hollow cylinder d ef, of the same 
height. The vessel DEF is filled with a weak acid solution. 
We introduce into the opening d ef& cork stopper, through which 
passes firmly a large copper wire PH, fitted to raise or depress 
the cup H, in which rests the point 5 of the movable conductor 
s k p, connected at p with a copper circle p Z, the support I k 
being of some non-conducting substance. The wire PH com- 
municates at P with one of the extremities of the voltaic appara- 
tus, for example, with the vitreous. To the vessel DEF we 
solder another wire MRTUJV, which makes several turns about 
DEF for the purpose of increasing its energy ; it then returns to 
U, near the first wire, and thence passes to the other extremity of 
the voltaic apparatus. 'When the communication is established 
the current mounts through the wire PH, descends by the 
movable conductor, traverses the liquid, passes through the wire 
MRTUJV, and produces a continued motion, in the direction v z, 
of the radius k p of the movable conductor. If both branches 
k I, k p, were conductors, the velocity of rotation would be 
doubled. 

The currents which traverse the liquid have no influence upon 
the rotation; for, if only the branch kp were a conductor, the 
current p x I would be in a direction the reverse of that of p y I. 
If both the branches kl,k p, were conductors, there would be in 
the two semicircles two opposite currents. Thus the actions 
produced by the currents of the liquid destroy each other. 

If we raise the cup H, and substitute for the movable conduc- 
tor that represented in figure 142, the same motion of continued 
rotation will still take place ; and it is proved by experiment, that 
this would be precisely of the same force. But, if we put in the 
cup H the point 5 of the conductor s p h i k, whose other ex- Fig. 143. 
tremity k returns to the axis of rotation, and carries a cup o, con- 
taining mercury, and communicating with the resinous extremity 
of the voltaic apparatus, this new conducter, whose two extremi- 
ties are in the axis of rotation, would no longer turn by the 
action of the wire MRTUJV. 

The direction in which the movable conductor, above referred Fig. 142. 
to, turns about the point A, changes when the direction of one of 
the currents is reversed. 

E. fy M. 40 



314 Electro-Dynamics . 

Closed Currents and Solenoids. 

261. The voltaic apparatus and the conductor of this instru- 
ment which makes the communication between its extremities, 
form necessarily a closed current, or rather an assemblage of 
closed currents, which are only more distant from each other in 
the voltaic apparatus than in the wire. When we render a part 
of this assemblage of currents movable, if the two points of un- 
ion of the movable part with the rest of the circuit are very near 
to each other, this movable part is what we call a closed current, 
although the current which passes through it is not strictly so, 
since it is always necessary that there should be a certain distance 
between its two extremities. 

That we may have a just idea of the motion which this part 
must take by the action of a fixed conductor, it is necessary to 
consider the action which this conductor exerts upon each of the 
parts composing the closed current; which is 'attended with no 
difficulty after what precedes. We shall content ourselves with 
giving an example. 
Fig. 144. Let there be a movable conductor of a rectangular form ; if 
the points x, y, are placed in the cups communicating with the two 
poles of the voltaic apparatus in such a manner that the point x 
shall be in the vitreous cup, the current will take the direction 
x a b c d efy. 

Let us examine the action which will be produced upon the 
movable conductor by an indefinite current MJV. We will sup- 
pose that the direction MJV is at first perpendicular to the plane 
of the rectangle, of which the parts a b, c d, ef, are horizontal, 
while the two branches b c, d e, are vertical. Let us draw through 
MJV a vertical plane p q o, cutting a b, c d, in any two points 

This being supposed, the ascending current d e is directed to- 
258. wards JV by the fixed current MJV. The descending current b c is 
carried in the opposite direction, that is, towards M, so that the 
rectangle be ed will turn about the vertical axis passing through the 
cups x, y. The horizontal parts dq, c q, tend also to place them- 
selves parallel to MJV. Lastly, the portions ef, a b, tend to pro- 
duce effects opposite to those just described ; but these effects 
are too feeble, on account of the distance, to counterbalance the 



Closed Currents and Solenoids. 315 

first. Thus the movable conductor will be brought into a plane 
parallel to MJY. 

When the moveable conductor is in this plane, the effects pro- 
duced upon the vertical branches be, de, destroy each other; for 
the force which MJY exerts upon b c, d e, are always parallel to its 
direction, since the currents do not cease to be perpendicular to it 
in the motion just described. They are, moreover, opposite to each 
other; and it is evident, besides, that they are equal when their plane 
is parallel to MN. In virtue of gravity, the plane of the rectan- 
gular movable conductor tends to a vertical position ; it does not 
take it strictly, because the two horizontal branches, parallel to 
MJY, still experiencing the influence of this current, the inferior 
tends to approach, while the superior recedes from it. As the 
first is nearest to the indefinite conductor, it follows, that the plane 
a b c d ef w\\\ incline a little towards this conductor. We can ren- 
der it strictly vertical, by placing the axis of rotation x y above 
MJY. 

When the movable conductor occupies the position of equilib- 
rium to which the indefinite conductor tends to bring it, the infe- 
rior horizontal part is evidently directed from M toward JY, accord- 
ing to the course of the fixed current. If the current c d were in 
a direction contrary to that of MJY, the equilibrium would not be 
stable ; and when we come to change the direction of the current 
of the movable conductor, without altering that of the fixed con- 
ductor, it will be seen, that the first will immediately^make a semi- 
revolution, and will turn in such a manner that the inferior part 
will be in the direction JYM and not in the direction MJY. 

262. It is important to observe, that all the circumstances of the 
motion of the movable conductor are due especially to the action 
of the vertical branches. It is by their influence, that it places 
itself in a plane parallel to MJY; the effect of the horizontal cur- 
rents is merely to produce a slight inclination toward MJY, of the 
plane of equilibrium, when the closed current is not in the vertical 
plane passing through MJY. Now it is easy to extend to any 
closed conductor the results we have just obtained by reasoning, 
and verified by experiment, for a rectangular movable con- 
ductor. 

Let us substitute, for the rectangular conductor, above employed, 
any closed plane conductor upon which the current MJY is made Fig. 145. 
to act. Let m n be an element taken at pleasure upon the closed 



316 Electro- Dynamics. 

circuit ; through the extremities m, w, of this element draw the 
vertical line n p, and the horizontal line m p. For the small cur- 
rent mn maybe substituted its two projections mp,np, forming a 
sinuous current m np, which departs very little from the former 
and terminates at the same extremities. The same consideration 
may be applied to all the elements of the closed circuit ; and thus 
for this circuit may be substituted a series of vertical and horizon- 
tal elements, to which we can apply all that has been said of the 
vertical and horizontal branches of the rectangular and movable 
conductor. Here the action of the indefinite current MN upon 
any closed movable conductor, tends to bring this conductor into 
a position of stable equilibrium, in such a manner that its plane 
shall be parallel to MN, and that, moreover, the current of the in- 
ferior part shall be in the direction MN. 

If we imagine a movable system composed of closed currents 
whose planes are parallel to each other, and if we regard this sys- 
tem as having all its parts connected invariably together, the action 
of the current MN upon such an assemblage may be conceived to 
tend still to bring all the planes of which it is composed into a di- 
rection parallel to MN. This is sufficiently evident, since the 
individual actions, exerted upon the several planes conspire to pro- 
duce this effect, it being well understood, that all the currents are 
supposed to be directed the same way. 

Let us imagine, for example, that the currents are circles, assem- 
bled at equal distances upon the same axis, passing through their 
centres, perpendicularly to their planes. The system thus ob- 
tained has the form of a cylinder, and takes the name of an elec- 
tro-dynamic cylinder. Now all that we have said above, is clearly 
applicable to an electro-dynamic cylinder; an electro-dynamic cy- 
linder is therefore brought, by the action of an indefinite conductor 
MN, into such a position, that all the planes of the currents, of 
which it is composed, are parallel to MN. 

It is even in this case very easy to destroy completely the action 
of the horizontal elements of the system, so that there will remain 
for consideration only the action of the vertical elements. In 
Fig. 145. order to comprehend this, we should recollect, that, for each ele- 
ment mn of the closed current, we can substitute the horizontal 
projection mp, and the vertical projection np; the actions of the 
horizontal parts mp will destroy each other, if we suspend the cy- 
linder above MN, at its middle point, and place it in such a situa- 



Closed Currents and Solenoids. 317 

tion, that the axis shall be perpendicular to MJY. For, on each 
side of MJY there will be on the axis two circles equidistant, 
and by taking upon these two circles two horizontal homologous 
elements, it will be seen, that these two elements exert equal and 
contrary actions; there will only remain, then, those of the vertical 
elements which tend to give to the movable conductor the posi- 
tion assigned, if it does not already occupy it. 

In short, an electro-dynamic cylinder is in a state of equilibrium 
about a horizontal indefinite conductor, which is perpendicular to 
its axis, and which is symmetrically situated with respect to the 
two extremities of this axis. In order that the equilibrium maybe 
stable, it is further necessary, according to what precedes, that 
the currents of the inferior parts of the circles should be in the di- 
rection of the currents of the fixed conductor ; it is unstable in the 
contrary case, as is shown by experiment. 

263. We are naturally led by the above course of reasoning to 
the subject of solenoids. This name is given by M. Ampere to 
a system of small closed currents, equal and equidistant, the 
planes of which are perpendicular to any given line, straight or 
curved, in which their centres are situated, and which serves as 
the axis of the solenoid. It will be seen, that the electro-dynamic 
cylinder, whose effects we have been examining, is only a partic- 
ular case of the solenoid, namely, that in which the axis is a 
straight line. Reciprocally, we shall soon see, that for any solen-^ 
oid whatever we may always substitute an electro-dynamic cylin- 
der, constructed upon the straight line joining its two extremities. 
The remarkable properties which have been recognised in the 
cylinder apply thus to any solenoid whatever. 

In order to construct a solenoid upon a given axis AB, it is Fig. 146, 
necessary, according to our definition, to divide this axis into 
equal portions, then through the points of division to draw planes 
perpendicular to the curve, and to trace in each plane about the 
point where it cuts the axis AB a small constant closed curve, for 
example a small circle, having this point for its centre. It may 
appear difficult at first to construct a solenoid that shall verify by 
experiment the results of the calculus ; we may succeed, neverthe- 
less, and by a very simple process, by bending a wire in the fol- 
lowing manner. We form, in the first place, a circle ah c nearly Fig. 147. 
complete ; then we carry the wire in a small straight line c d, per- 
pendicular to the plane of the circle ; we next form a second 



318 Electro-Dynamics. 

circle d ef, and another straight Ymefg, in the direction of the 
first ; and so on, to the extremity A, at which we stop, and from 
which we make the wire return in a straight line in the direction 
AB, opposite to c d,fg, Sic, taking care, at the same time, 
that the wire AB shall be as near as possible to c d,fg, he, 
from which it ought not to be separated except by the silk which 
covers it. When an electric current is made to pass through such 
a system, it is evident, that the effects of the parts c d, df, &c, are 
destroyed by the equal effects of the part AB which comes from 
the opposite direction. There will remain, then, only the effects of 
the circles a be, d ef, he, which are all equal to each other, and 
perpendicular to the same straight line AB, and consequently 
compose a solenoid. 

For a solenoid we may sometimes substitute a continued helix, 
that is, a wire bent into the form of a spiral, from one extremity A 
to the other extremity B, and returning in a straight line from B to 
A, in the direction of the axis of the helix. Since, for each ele- 
ment of the helix, may be substituted a small portion of the gen- 
eratrix of the cylinder, and a small portion of the arc of a circle, 
forming a continued circuit and terminating in the two extremities 
of the element ; and, if we add together the several parts of the 
current situated upon the generatrix, which are obtained by this 
substitution, it will be seen, that their whole effect is sensibly de- 
stroyed by the opposite effect of the rectilineal wire which is made 
to return from B to A in the interior of the helix. There will 
remain, then, the successive elements of the circular arcs, whose 
action must in fact differ very little from that of a solenoid hav- 
ing AB for its axis ; but, although helixes have been often em- 
ployed as solenoids, it is manifest, that it would be better to make 
use of the more exact, as well as more simple apparatus above 
described. 



Reciprocal Action of Solenoids. 

264. Before making known the results to which the calcujus 
has led, relatively to the mutual action of two solenoids upon each 
other, and of a solenoid upon an element of a current, it may be 
well to give the names which are applied to the extremities or 
poles of this solenoid. We shall see, hereafter, that solenoids, like 



Reciprocal Action of Solenoids. 31 9 

magnets, are directed by the action of the earth, so that one of 
their poles is carried toward the north pole of the earth. We 
give the name of austral pole to that extremity of the solenoid 
which is directed toward the north, and the name of boreal pole 
to that which is directed toward the south. 

We resume now the analysis of the properties of the solenoid. 
By performing the experiment, the theory of which is given in 
article 261, we shall find, that a solenoid, in which the line joining 
the poles is horizontal, is directed by an indefinite rectilineal con- 
ductor, placed under its middle point, in such a manner that this 
straight line makes a right angle with the direction of the conduc- 
tor, and that is austral pole is to the left of the current ; that in this 254i 
situation it attracts the solenoid, and that it repels it when the aus- 
tral pole is to the right of the current. We thus find, that in two 
solenoids, the poles of the same name repel each other, and the 
poles of opposite names attract each other, precisely as in the 
case of magnets. All these facts, derived from experiment, are 
deduced from the calculus. But the calculus leads besides to 
various theorems, which it would be difficult to discover by obser- 
vation ; and which are all founded on the general formula by 
which M. Ampere represents the mutual action of two elements 
of a conducting wire. 

We shall now give the different theorems in the order in which 
they naturally present themselves. 

The first fact consists in this, that the contiguous portions of the 
same solenoid attract each other reciprocally. If, therefore, we 
consider a rectilineal solenoid, or an electro-dynamic cylinder, 
indefinite in both directions A and B, and suppose that this 
solenoid is divided into two parts by a plane perpendicular to its Fig. 146. 
axis, and that those two parts are separated from each other in 
the direction of this straight line, we shall obtain two solenoids 
A' B and A" B", indefinite each in one direction, and which 
necessarily attract each other ; since, by taking a circle upon each, 
the currents of these two circles proceed the same way, and can 
only produce attraction. It will moreover be readily seen, that 256. 
the extremities A', B", of the two solenoids will be of contrary 
names ; for in order to place them beside each other, in such a 
manner as to render them indefinite in the same direction, it will 
evidently be necessary to invert one of them ; their currents, 
which actually proceed in the same direction, would therefore 
move in opposite directions after the inversion. 






320 Electro-Dynamics. 

It will be easily perceived, that the reciprocal action of the two 
indefinite solenoids, situated as above described, in such a man- 
ner that their axes shall be in the same straight line A' B", is 
directed according to this straight line ; since this takes place for 
the individual actions of the two circles, taken upon the two 
solenoids, on account of the symmetry of the figures. The 
calculus shows, moreover, that the value of this action is in the 
inverse ratio of the square of the straight line B' A", which joins 
the poles. This force, nevertheless, does not emanate from the 
poles themselves, but from the whole of the solenoids. It is the 
resultant of the forces produced by all the circles of the one upon 
all the circles of the other. So that the action of the poles of a 
solenoid is an abridged expression. 

If we now make the two solenoids to turn in any manner about 
their poles B'. A", supposed to be fixed, and if we give the axes 
any curvature whatever, provided we do not alter the distance 
or magnitude of the circles of which they are composed, the 
calculus shows, that their reciprocal action remains the same, 
notwithstanding all the changes of form and position. 

We change the poles of contrary names to poles of the same 
name by reversing the direction of. the current in one of the 
solenoids, which renders the action repulsive, without changing 
255. the energy, conformably to one of our first experiments. 
Fig. 148. A definite solenoid AB may always be considered as the differ- 
ence of two indefinite solenoids ABC, BC, having their currents 
directed opposite ways, and of which the part BC of the first 
completely coincides with the second. For it is evident, that the 
opposite effects produced by the currents of this common part 
destroy each other, so that the solenoid AB, and the two indefinite 
solenoids ABC and BC, may be considered as one and the 
same thing. 

Hence the action of a definite solenoid depends simply on 
the position of its extremities, and is altogether independent of 
the figure given to the axis between these two points ; hence, also, 
the actions assigned to its poles are of contrary signs. For, the 
poles of two indefinite solenoids, which we substitute for a definite 
solenoid, are necessarily of opposite names, since the currents 
of the two solenoids are in opposite directions. Moreover we 
have just seen, that the action of an indefinite solenoid depends 
simply on the position of its pole. Therefore the action of the 



Action of Solenoids upon Conducting Wires. 321 

two indefinite solenoids, substituted for the solenoid AB, will 
remain invariable, whatever be the manner in which we bend the 
axis of AB between its points A and B, supposed to be fixed. 
We may therefore always substitute, as we have advanced above, 
for any solenoid whatever a cylinder having the same poles, and 
whose axis shall be the straight line joining those poles. 

It will be readily inferred from what precedes, that the action 
of two solenoids AB, A' B', is reduced to four forces, directed Fig. 149. 
according to the straight lines A A', BB', AB', BA', the first 
two being repulsive, because they take place between poles of the 
same name, and the last two attractive, because they are exerted 
between poles of opposite names ; each of these forces having a 
value reciprocally proportional to the square of the corresponding 
distance. 

Another theorem, demonstrated by the calculus and which may 
be regarded as a corrollary to the foregoing, is that which relates 
to closed solenoids. It is as follows. If the extremities of a solenoid 
AHB are united in such a manner as to result in a closed sole- 
noid A' H' B', this solenoid will be destitute of any action either Fig. 150. 
upon another solenoid or upon an element of the conducting wire. 

Action of Solenoids upon Conducting Wires. 

265. We have spoken hitherto only of the action of solenoids 
upon each other ; but we must also consider the action of sole- 
noids upon conducting wires, and particularly upon an element 
of the electric current. As we have furnished the means of sub- 
stituting in all cases for the action of a definite solenoid, that of 
two indefinite solenoids, we are able to limit ourselves to the 
examination of the action of an indefinite solenoid AB upon an Fig. 151, 
element m m' of the current, and may even substitute for this 
solenoid an electro-dynamic cylinder AC, whose axis is the 
prolongation of the straight line IA, drawn through the given pole 
A to the middle of the element m m'. 

It will be seen, therefore, that it is proposed to determine the 
action exerted by an indefinite electro-dynamic cylinder AC, 
upon an element m m' whose middle point lis in the prolongation 
of AC. This element may make with the straight line AI any 
given angle ; we shall first examine the two simple cases in one 

E. fy M. 41 



322 Electro-Dynamics. 

of which it is directed according to the axis of the cylinder, and 
in the other perpendicular to this axis. 

Fig. 152. In the first case, represented in figure 152, it is easily proved, 
that the action produced is nothing between the solenoid and the 
element m ml, and it is even nothing between this element and 
an infinitely small arc p p', taken at pleasure upon any one of the 
circles of which the cylinder is composed. For if, through the 
middle of this arc p p' and the axis of the cylinder, we draw a 
plane, this plane will be perpendicular to the arc. Therefore the 
small current p p', whose action upon m ml is in question, is such 
that m m! is found in a plane perpendicular to its middle point. 
Now it may be shown, that any infinitely small current has no 
action upon another current m ml, which passes in a plane perpen- 
dicularly to the middle of the first. 

Fig. 153. i n or der to prove this, let there be two equal elements MM, 
JVJV, situated upon a straight line MOJV, perpendicular to a 
plane PQ, equally distant from the point O, and both tending 
toward O. If the two currents MM, m ml, approach the foot 
of their common perpendicular OK, it is evident that the same 
thing will take place also with respect to JVJV, m ml. Therefore 
the actions of MM, JVJV, upon m ml are of the same sign ; 
they are also equal on account of the equality of the lengths 
MM, JVJV, and of the distances MO, NO. If now we imagine 
that MM and JVJV' are carried along MOJV with a uniform 
motion till they meet each other, and that, after reaching O, they 
pass this point, the actions which they exert upon m ml will not 
cease to be equal and of the same sign, since they will have both 
changed their signs ; so that the same must hold true also at the 
moment of their passage, when their middle points coincide with 
O ; but then it is evident, from an experiment of which we have 
often made use, that the two actions, if they exist at all, would 
be equal and of contrary signs, since the two currents MM, 
JVJV', have opposite directions. It follows, therefore, that each 
is nothing. Moreover, when the two elements MM, JVJV, 
have their middle point in O, the plane PQ in which m to' is 
situated is perpendicular to them, and passes through their middle 
point. Therefore an element of a current exerts no action upon 
another element, situated in a plane perpendicular to the middle 
of the first ; hence also the element m ?n', in figure 152, is desti- 
tute of any action upon p p ', and, consequently, upon the cylin- 
der AC. 



Action of Solenoids upon Conducting Wires. 323 

We pass now to the case represented in figure 154, in which Fig- 154. 
the line 1A is perpendicular to m m' . Suppose, for example, that 
the plane AI m' is horizontal ; draw through the axis CAI oi the 
cylinder a vertical plane, which will of course be perpendicular to 
mm'. We divide thus the solenoid into four parts, which will 
exert upon m m' different actions, which it is proposed to analyze. 

The case will be simplified by considering only one of the 
circles of the solenoid ; and we shall prove that the resultant of 
the actions of this circle upon the element is perpendicular to the 
plane Al m! m. 

In figure 155 is represented the four quadrants corresponding Fig. 155. 
to the several parts of the solenoid ; by taking, in those quadrants, 
four elements equal to each other and similarly situated a a', b V , 
c c', d d', it will be readily seen, that the two a a', d d' attract the 
given element, while the two b b', c c', repel it with the same 256. 
force. 

If, therefore, the element m m' were situated exaetly in the 
plane of the circle, the repulsive force c c' would be added to the 
attractive force a a', and would thus double the effect. The re- 
pulsive force b b' would double, in like manner, the attractive 
effect of d d'. The attractive forces then would result in a single 
force, directed according to 1L, perpendicular to m m' , and 
would tend from J toward L. The same may be said of the 
repulsive forces. 

If we conceive the element m m' placed, as it ought to be, a 
little out of the plane of the circle, so that the figure shall repre- 
sent its projection, it will be seen, that the four partial actions will 
still result in a single action tending in the direction 1L, (the 
components, directed according to a perpendicular to the plane of 
the circle destroying each other,) and which will be directed 
according to a straight line passing through the middle of m m', 
perpendicularly to this element, and parallel to the plane of the 
circle, since the four equal component forces are. evidently dis- 
tributed in a symmetrical manner about the straight line, which 
does not differ from the perpendicular drawn through the middle 
of the element to the horizontal plane Aim', just constructed. 

It is proved, therefore, that, when the element m m' is in a 
direction perpendicular to the axis of the cylinder, the action of 
the cylinder upon this element is reduced to a single force pass- 
ing through the middle of m m', and perpendicular to the plane 



324 Electro-Dynamics. 

of the sector, having for its base m m', and for its vertex the 
pole A. 
Fig. 156. We shall now consider the general case in which the element 
m m! makes any angle with the axis of the cylinder ; but this case 
reduces itself to the two preceding ; for, if we project m m! upon 
the axis in question, and upon a perpendicular to this axis, drawn 
in the plane A m m', we may substitute for the element m m! its 
two projections, which evidently compose a small sinuous current, 
departing very little from m m', and terminating at the same points 
m, m'. In this figure LIU represents the perpendicular to the 
axis CAI of the cylinder, drawn in the plane A m ?ri ; m n, m! n' 
are two perpendiculars to this straight line, n n' is, therefore, 
one of the projections of m m' under consideration, and the two 
small straight lines mn, m' n', compose the other. For the ele- 
ment mm' may thus be substituted, (1.) the two small currents 
m n, m' n', which may be regarded as acting in I along the axis 
of the cylinder, to which they approach infinitely near, and to 
which the direction is parallel ; so that, according to what is above 
shown, the cylinder has no action upon them ; (2.) the current 
n n' ', perpendicular to CAI, upon which the cylinder acts with 
a force perpendicular to the plane Aim. 

In short, therefore, the action of a cylinder or rather the action 
of any indefinite solenoid, having its pole at the point A, upon 
an element m m', reduces itself to a force perpendicular to the 
plane of the sector A m m', which passes through the middle 
point I of m m'. 

As to the intensity of this force, it may be inferred from the 
preceding construction, that for the same distance AI it is pro- 
portional to the sign of the angle Aim', since the action of the 
element m m' is represented by its projection n n', and we have, 
by the doctrine of projections, 

m m' sin. / m! n' , or m m' sin. AI m' =. n n'. 
Hence the general rule, as demonstrated by geometers ; the 
force is always proportional to the sector A m m', having for its 
vertex the pole A, and for its base the element m m', and in the 
inverse ratio of the cube of the distance AI. 

Such are the principal results obtained by the calculus rela- 
tively to the action of solenoids upon each other, and of a sole- 
noid upon an element of the current. The reader can scarcely 
have failed to remark the singular analogy thus established be- 



Law respecting the Intensity of Currents. 325 

tween solenoids and magnets ; indeed, we shall soon see, that, by 
regarding magnets as solenoids, we are able to explain magnetic 
phenomena, and thus to reduce to a single cause these phenom- 
ena and those of electricity. 



Law respecting the Intensity of Currents. 

266. MM. Biot and Savart have sought by experiment the law 
according to which the action of a current varies vviih the change 
of distance. In this inquiry they made to oscillate, for a given 
time, a very short magnetic needle, presented to the action of a 
vertical current, the distance of which was varied, the length of 
the wire being so great that its extremities might be considered 
as having no action upon the needle. This disposition represents 
an indefinite wire. The needle, rendered astatic by the presence 
of a magnet placed in the magnetic meridian, would place itself, 
after a certain number of oscillations, in a direction perpendicular 
to that of the current, since the two vertical parts of the wire 
would exert equal actions upon the needle ; hence the resultant 
of these actions must be in a horizontal plane. Having found 
by experiment, that the different oscillations for the same position 
of the current are isochronous, we infer, that the force which urges 
the needle is necessarily proportional to the angle of deviation 
from its final direction. It hence results, that the formula for the 
pendulum is applicable to this case, and that consequently the 
force of the current is proportional to the square of the number M S} an * 
of oscillations made in a given time. Making the calculation 
according to the results of our experiments, we find the whole 
force of the current upon a magnetic element, austral or boreal, 
is in the inverse ratio of the distance of this element from the 
current. 

This law relative to the total resultant of the magnetic ele- 
ments of the needle and of the current being known, it w T as 
shown by M. de la Place, as a necessary consequence, that the 
action of each element of the current upon the magnetic element 
is in the inverse ratio of the square of the distance. But as the 
law relative to the distance may be modified by the direction of 
each distance with respect to the general direction of the wire, it 
remained to show, whether the coefficient really existed, and what 



326 Electro-Dynamics. 

was its composition. To ascertain this, MM. Biot and Savart 
arranged the experiment in the following manner. The two 
Fig. 157. parts mo, o n, being equally inclined to the horizon, the distance 
of the point o from the needle being constant, the angle m o n 
being made to vary in each experiment, the action of the bent 
wire m o n, and of the similar rectilineal wire m' o n', were ob- 
served alternately. The experiment showed, that the total action 
varied in proportion to the tangent of half the angle m o F, or to 
the tangent of a fourth of the angle formed by the two wires ; 
whence we infer, that the action of an element of the current upon 
a pole of a magnet, or upon an austral or boreal element, varies 
in proportion to the sine of the angle made by the direction of 
the current with the line, which joins the pole of the magnet, or 
the magnetic element, and, the middle of the element of the cur- 
rent. Whence it will be seen, that the law of the action of an 
element upon the pole of a magnet, obtained by experiment, is 
the same as that of an element of the current upon the extremi- 
ties of a solenoid deduced by the calculus from the formula which 
represents the mutual action of two elements of the current. 



Mutual Actions of Magnets and Currents. 

Fi 26< i4i %67. In the experiment described above, in which the con- 
ductor k p turns with a continued motion about the axis GH, 
by the action of a fixed conductor, carried in a circle about a 
vessel DEF, for this fixed conductor we may substitute a ver- 
tical magnet, the currents of which move in the same direction. 
If, for example, the current in the fixed circular conductor move 
in the direction north, east, south, west, we may substitute for it 

Fig. 158. a magnet AB, situated as represented in figure 158, in such a 
manner that the austral pole A of the magnet shall be downward ; 
for then, according to what is above laid down, the currents of 
the magnet proceed also in the direction north, east, south, west. 
If the austral pole were upward, the motion would take place in 
the opposite direction. We suppose here, as in the former ex- 
260. periment, that the current descends in the movable conductor. 
The action of the magnet, like that of the fixed conductor, re- 
mains the same upon the movable conductor, whatever be the 
magnitude and form of this last, provided the two extremities are 



Continued Revolution of a Magnet. 327 

always at the same points, and, like the fixed circular current, the 
magnet will not produce a continued revolution when the movable 
conductor has its two extremities in the axis GH, or when it 
forms a closed circuit. 

The same effect takes place when we incline the magnet by- 
giving it the position represented by A' B'. The pole B' re- 
maining always in the axis, we may even place the magnet in a 
horizontal position A" B" . In this last case, however, it is only 
the superior parts of the currents of the magnet which tend to 
turn the movable conductor in the direction in which it actually 
moves ; the currents of the inferior part tend evidently to move 
it in the opposite direction, and it is only the difference of these 
two actions, which determines the motion. To render it more 
rapid, we place below the vessel DE magnets, arranged like the 
rays of a star, in such a manner that they shall all have their 
extremities of the same name in the axis GH. 

These different experiments may be performed without the vol- 
taic apparatus. It is sufficient for this purpose, that the vessel EDvig. 159. 
should be of zinc, as well as the cross-piece OVED, which is sol- 
dered to the copper rod GH, that carries the cup H, containing 
the point s. The electro-motive action, exerted in G, produces 
a current which passes from the zinc to the acid solution in the 
vessel, and thence to the movable conductor ; the current of this 
conductor is therefore always ascending, producing a motion, con- 
sequently, in a direction north, east, south, west, when the boreal 
pole of the magnet AB is upward, or in the axis, when the mag- 
net is horizontal. 



Continued Revolution of a Magnet by a Current about a Line 
parallel to its Axis. 

268. We employ for this experiment the instrument represented 
in figure 160. AB is a magnetic bar, ballasted by a platina bar 
AD, in mercury, with which the vessel is nearly filled. A con- 
ductor c d enters the mercury in the middle of the vessel ; the 
other wire of the voltaic apparatus communicates with a ring m n, 
introduced also into the mercury. The magnet will turn as soon 
as the voltaic apparatus is put in action. 

In order to understand this experiment, it must be remarked, 



328 Electro-Dynamics. 

that on the upper part of the mercury a great number of currents is 
produced, of which some are exterior to the surface of the mer- 
Fig. 161. cury, while others traverse it. Let a b c be a section of the ring 
m n, r r' a section of the magnet, and V \he centre of the currents. 
Suppose these directed from the centre to the circumference, and 
that the austral pole of the magnet is immersed in the mercury. 

Let us consider the two currents VA, VA! ; the current VA 
attracts the portion of each current of the magnet whose convexity 
is turned towards it, since the motions take place in the same 
256. direction ; and repels the other parts, but with a less force, on ac- 
count of the greater distance. The current VA, on the other 
hand, attracts the remote currents and repels those which are near. 
The attractive action of the first is directed according to r 1 x, and 
the repulsive action of the second according to r x. Consequently 
the resultant of these two forces will be in the direction TT, per- 
pendicular to V x, and will push the magnet in the direction TjP. 
In like manner, the two currents VB', VB, have a resultant in 
the same direction TjP; thus the magnet will describe a curve 
perpendicular to V x. that is, a circle. 

As to each of the currents which traverse the magnet, we may 
divide it into three portions ; one from the point V to the mag- 
net, another in the interior of the magnet, and the third from the 
magnet to the ring a b c. The second portion, producing only 
reciprocal attractions and repulsions between the different points 
of the magnet, can cause no motion. But, if we examine the 
action of the first and last portions with respect to V x, we shall 
find, that the resultant of these actions is directed according to 
258. x T. Thus the action of the currents which traverse the mer- 
cury tend to turn the magnet about the point V in the direction 
x T'. If the magnet were covered with an insulating substance, 
that would prevent the electric current from traversing it, the sec- 
ond portion of the currents above mentioned, and which exerts no 
action, would be transferred to the mercury, and its action would 
evidently conspire with that of the two other portions in pushing 
the magnet in the same direction, which would produce no 
change in the direction of the motion, an,d would only render it a 
little more rapid. We shall not here consider the action of the 
rest of the voltaic current, since its distance from the magnet 
is such that its effect may be neglected. 

An objection may be raised to the preceding explanation, which 



Rotation of a Magnet. 329 

is easily answered. It is well known, according to the doctrine of 
M. Ampere, that the action of two closed circuits, or of two sys- 
tems of closed circuits, cannot impress upon one of them a mo- 
tion of continued rotation in the same direction. Now, in the 
preceding phenomenon, all the currents which traverse the mer- 
cury, without meeting the magnet, make a part of a closed circuit ; 
thus the rotation would not take place, except from interrupted 
circuits which traverse the magnet, and if the magnet were cover- 
ed with an insulating varnish, the rotation ought not to take place 
at all, which is contrary to fact. But it is to be remarked, that 
the principle of M. Ampere is applicable only to currents and 
systems of currents, all the parts of which are invariably connected 
together, and that it would not hold true if any one part of the 
system were liquid. 



Rotation of a Magnet about its Axis by the Action of a Current. 

269. Let AB represent a magnet, the top of which contains a Pi S- * 62 - 
cup filled with mercury, destined to receive one extremity of the 
voltaic conductor, while the other extremity communicates with 
the ring which surrounds the mercury. As soon as the voltaic 
apparatus is put in action, the magnet turns rapidly upon itself in 
a direction opposite to that of its currents, if the current of the 
voltaic apparatus is directed, in the magnet, from the top down- 
ward. The direction of the motion will be reversed if we change 
either the poles of the magnet or the direction of the voltaic cur- 
rent; and it will remain the same if we change, at the same time, 
both the poles of the magnet and the direction of the current. 

This phenomenon is explained in the following manner. Let CD Fig- 163. 
be the section of the magnet at the level of the mercury, AB its 
axis, and ab cd a section of the ring which surrounds the mercury. 
We indicate the direction of the currents in the magnet by the 
arrow-heads. The currents which traverse the magnet will have 
no action upon it, as has been already shown, and it will be urged 
only by the currents which traverse the mercury, and by the rest 
of the circuit. This last part being much more feeble than the 
other, the direction of the rotation will be determined by the 
former. Let C a be one of these currents ; all the elements of 
this current attract the currents of the magnet which are to the 

E. fy M. 42 . 









330 Electro-Dynamics, 

256. right, and repel those which are to the left ; and, as all the elements 
of the magnet are, two and two, symmetrically placed with respect 
to the current C a, the resultant will be perpendicular to this last, 
and will have a direction opposite to that of the currents of the 
magnet, which is conformable to experience. We are able, more- 
over, to explain the same phenomenon upon the principle demon- 
strated by M. Ampere, that the action of a circular current is 
nothing upon a second circuit, the two extremities of which are 
in the axis of the first ; for the portion of the circuit exterior to 
the magnet produces an effect equal and opposite to that produced 
in the magnet by the portion of the circuit which traverses it, on 
the supposition that the first passes through the independent mol- 
ecules of the magnet ; this last action must make the magnet 
turn upon itself in the direction of the currents. Thus, as the 
contrary effect must take place, the magnet must turn in a direc- 
tion opposite to that of its currents. 



Rotation of a Voltaic Conductor upon its Axis by means of a 

Magnet. 

Fig. 164. 270. In the figure 164, m m is a horizontal copper rod, resting 
on the insulating pillar AB. To this rod is fixed a case L, intend- 
ed to sustain the magnet. The inferior part of the magnet termi- 
nates in a point, which enters the cup v, of an iron bar a 6, ballasted 
in the mercury by the platina weight b ; the other parts of the 
apparatus being the same as in the preceding experiment. This 
experiment becomes in all respects similar to that in which the 
267. magnet impresses upon a conductor a continued revolution. 

Rotation of Mercury by a Magnet. 

Fig. 165. 271. We introduce into the bottom of a vessel DE two polish- 
ed copper wires m n, m! n', of the same length, and covered with 
an insulating substance, except their extremities. The vessel 
being filled with mercury so as to cover the tops of the wires, the 
inferior extremities are made to communicate with the poles of a 
voltaic apparatus in action. As soon as the communication is es- 
tablished, the mercury rises above the wires, from which waves 



Action of the Earth upon Voltaic Conductors. 331 

proceed in all directions. If we bring near the pole of a powerful 
magnet, the mercury is depressed, and a rotation commences ; if 
the magnet is brought still nearer, a greater depression takes 
place. Tin in a state of fusion is found to answer the same pur- 
pose, and indeed all good liquid conductors. 

The two currents taking place in the wires repel those which 
are established from m to m', since they move in opposite direc- 
tions and are oblique to each other ; hence the mercury is elevat- 255, 
ed. If we bring a magnet near to one of the cones, it tends to 
make the mercury revolve, and the centrifugal force depresses the 266. 
cone which was formed. With a certain degree of velocity the 
surface becomes plain and even concave. 

Action of the Earth upon Voltaic Conductors. 

272. The earth gives a direction to magnets ; magnets act 
upon voltaic conductors. The earth may be expected to act up- 
on conductors. The following experiments will serve to illustrate 
this subject. 

A vertical current, movable about a vertical axis, is carried by 
the action of the globe to the east of this axis, when the current 
descends in the movable conductor, and to the west of the same 
axis, when it ascends. The plane which, in the position of equi- 
librium, passes through the current and through the axis, is in 
both cases perpendicular to the magnetic meridian. From this 
fact and that of art. 258, it is inferred, that, if the action of the 
earth is owing to electrical currents, they produce the same effects 
that would be produced by a mean current, directed in the globe 
from east to west. This fact may be established by the following 
experiment. 

The movable conductor x a b cd e y is composed of a copper Fig. 166. 
wire, bent as represented in the figure. Only one of the points 
x, y, touches the bottom of the cup in which it is placed, and it is 
upon this that the movable conductor turns freely, the part b c 
being rendered vertical by means of the counterpoise z. The 
arrow-heads indicate the direction of the current for the case in 
which the branch b c moves towards the east. The two horizon- 
tal branches a b, d e, having currents in contrary directions, con- . 
tribute nothing to the rotation. 






332 Electro-Dynamics. 

Fig 167. 273. A horizontal conductor which can only move parallel to 
itself is always (in the northern hemisphere) carried by the action 
of the globe to the left of an observer, supposed to be placed in 
the current in such a manner, that this current passes from his head 
to his feet, his face being turned toward the earth, whatever be 
the azimuth of the current. If, for example, the current is from 
east to west, it is carried to the south ; and, as it is necessarily 
attracted in this case by the mean current, equivalent to all the 
terrestrial currents, since this current proceeds from east to west, it 
follows, that this last is to the south of the place of observation. 

Each of the points x 9 y, enters a cup filled with mercury. We 
designate the direction of the current by supposing the point y in- 
troduced into the vitreous cup. It is evident, that the actions 
exerted by the earth, upon the two vertical branches x a, y b, de- 
stroy each other, and that the motion observed is produced by the 
single horizontal branch a b. We place the conductor in a plane 
sensibly vertical by means of the counterpoise *. In this experi- 
ment the branch a b is always carried to the left of the current, 
with the same force, in whatever azimuth the apparatus is placed; 
a result that is conformable to what is made known by the calcu- 
lus. If the current proceeds from the west to the east, it is 
repelled to the north by the terrestrial current. 

Fig. 168. 274. A horizontal conductor a c, movable about a vertical 
axis passing through one of its extremities, turns by the action of 
the earth with a continued motion, which takes place in the direc- 
tion east, south, west, north, when it passes from the circumfer- 
ence to the centre, and in a contrary direction when it goes from 
the centre to the circumference. It results further, from this ex- 
periment and that of article 258, that the mean current is to the 
south of the place of observation. In this instrument the circle is 
entire, and one or both of the radii may be conductors ; in ihe 
second case the velocity is doubled. The experiment is perform- 
ed like that of article 260. The currents in the liquid have no 
influence in this case, since every thing is symmetrical on the two 
sides of the radius of the circle. 

Fig. 169. 275. A conductor, movable about a vertical axis and forming 
a plain circuit, and nearly closed, takes, by the action of the 
earth, such a situation, that the part in which the current is de- 
scending is carried to the east, and that in which it is ascending, 
to the west. 






Astatic Conductors. 333 

The motions of the circle are in certain cases obstructed by this Fig. 169. 
mode of suspension ; but this inconvenience may be remedied by 
substituting for the circle, figure 169, the movable conductor, 
figure 170. The ring a b affords a passage for the conductor 
PUS, coming from one of the extremities of the voltaic appara- 
tus, and which supports the cup S, fitted to receive the point s, 
upon which the conductor is to turn. The small cup d, where 
terminates the copper wire of which the movable conductor is 
composed, and which sets out from the point s, contains mercury, 
in which is placed a copper wire d c, attached by means of a vice . 
b, to a conducting wire, that communicates with the other extremi- 
ty of the voltaic apparatus. This new mode of suspension allows 
the circle to turn in all directions; and, by changing alternately 
the direction of the current, we can even produce a motion of con- 
tinued rotation. 

276. If we form a helix with a conducting wire, and the hori-Fig. 171. 
zontal axis of this helix is capable of turning about a vertical 

line passing through its middle point, each ring of the helix directs 
itself like the circuit above described ; and the axis of this helix, 
which is sensibly perpendicular to the plane of its rings, will be 
directed from north to south, in the plane of the magnetic merid- 
ian, in such a manner, that the part of the helix in which the cur- 
rents descend are turned to the east and that in which they ascend 
to the west. 

Astatic Conductors. 

277. In making experiments on the mutual action of conduc- 
tors, the action of the earth upon the movable conductor is liable, 
when no precaution is taken, to affect the movable conductor, 
and may even disguise entirely the effects produced by the fixed 
conductor. Hence, M. Ampere was led to construct movable 
conductors, which he called astatic, and upon which the action of 
the earth is insensible, since they are composed of parts that are 
equal and symmetrical, upon which the terrestrial currents act in 
contrary directions, with equal forces; the great distance of the 
currents from the instrument preventing any appreciable differ- 
ence in these actions. The fixed current, on the other hand, 
being much nearer one of the two symmetrical portions than the 



334 Electro-Dynamics. 

other, exerts upon it a much greater action ; and we may easily 
observe the effect of the first of these two actions, which is only 
slightly altered by the effects due to the second. 
Fig. 172. Thus in the movable conductor, represented in figure 172, 
the motion of the electricity taking place in the direction x a b 
c d efg h i y, the current is descending in the part c d, and as- 
cending in the part g h ; they tend, therefore, to direct themselves 
one toward the east, the other toward the west, with equal forces. 
In the same manner also the action of the horizontal currents d e, 
h i, is counterbalanced by the action of the opposite currents b c, 
fg. As to the two parts a b, ef, since they are in the axis of 
rotation, they may be neglected. 

Accordingly, in order to render a movable conductor astatic, 
whatever be the figure which we propose to give to the part on 
which the fixed conductor is made to act, the most simple method, 
and that which is generally employed, consists in a construction by 
which we have in this conductor another portion equal to the first, 
and which the current traverses in an opposite direction. This 
may be seen in the movable conductors represented in figures 
173, 174, 175, 176. 



Of Magnetizing by Electric Currents. 

278. Soon after the discovery of M. Oersted was made known 
in Paris, M. Arago observed, that the conducting wire of the volta- 
ic apparatus attracted iron filings, like the magnet, and that these 
filings detached themselves, immediately upon the communication 
being interrupted. This phenomenon is not be attributed to the 
ordinary action of electricity, since the experiment does not suc- 
ceed with the minute parts of any substance not magnetic. More- 
over he perceived, that the voltaic wire communicated to the iron 
only a transient magnetism, while it imparted to steel a durable 
magnetism ; that the magnetizing takes place in a direction per- 
pendicular to that of the current, that is, in a direction which the 
needle tends to take when placed above or below the current ; 
and that two parallel steel wires, forming each a right angle 
with the conducting wire, and placed at equal distances from it, on 
opposite sides, acquire the same degree of magnetism. 

M. Arago, by placing a steel needle in the helix imagined by 



Of Magnetizing by Electric Currents. 335 

M. Ampere, obtained magnets, the poles of which were reversed 
by repeating the experiment with a current in the opposite direc- 
tion. By introducing a steel wire into several helixes, formed with 
the same conducting wire, turned alternately in opposite directions, 
he produced consecutive points. Placed without the helixes, the 
steel wires were magnetized with difficulty. They ought also in 
this last case, to become magnetized in a reverse manner. M. 
Arago performed these experiments at first with a continued cur- 
rent ; he showed afterwards, that common electricity, transmitted 
by sparks through the same helixes, in like manner magnetized the 
steel wire. 

Mr. Sturgeon, of Woolwich, M. Moll, Professor Henry, and 
M. Eyke, have by this process produced artificial magnets capable 
of supporting a ton weight. 

279. All these particular facts are reduced to one general one, 
namely, that a steel needle, not magnetic, placed in a solenoid, 
traversed by a momentary electric current, presents, when separ- 
ated, precisely the same attractive and repulsive actions which the 
solenoid itself would have, with respect either to a conducting wire, 
or another solenoid, or the terrestrial globe ; and that it is on ac- 
count of these properties, that the needle becomes a true magnet, 
We are thus led to believe, that the magnets owe all their proper- 
ties to electric currents turning about their molecules, as they turn 
in a solenoid ; and that the union of all these currents, forming the 
magnet, acts like a solenoid which has its extremities at the two 
poles of the magnet. This theory, which the general fact above 
stated renders highly probable, acquires all the validity of which 
physical theories are susceptible, when, by calculating from the 
formula by which M. Ampere represents the action of two ele- 
ments of the conducting wires, the action which a solenoid ought 
to exert either upon another solenoid, or upon an element of the 
conducting wire, we find precisely the same laws which were dis- 
covered experimentally by Coulomb for the mutual action of two 
magnets, and by MM. Biot and Savart for the action which a 
magnet and an element of the conducting wire exert upon each 
other. For these calculations we refer the reader to the works of 
M. Ampere upon this subject, and particularly to that entitled 
" Theories Mathematiques des Phenomenes electro-dynamiques, 
uniquement deduites de V Experience" 

280. There are two ways by which magnetizing by the electric 
current may be explained. 



336 



Electro-Dynamics. 



We may suppose that in the molecules of soft iron, the currents 
being directed all manner of ways, the exterior effect is nothing. 
According to this hypothesis, the voltaic current only gives to all 
the molecular currents a common direction. Or we may suppose, 
that the currents did not preexist in the iron, and that the voltaic 
current or the presence of a magnet gives rise to them. 

In order to satisfy ourselves whether this second hypothesis is 
admissible, we must ascertain if a voltaic current is capable of 
Fig. 177. producing other currents. For this purpose, M. Ampere suspend- 
ed a copper ring by a silk thread, and enveloped it in a spiral, 
similar to that of figure 178. The ring soon acquired the prop- 
erty of being put in motion by a magnet. Thus either hypothesis 
is admissible. 

Dr. Faraday has given new importance to the subject of cur- 
rents produced by influence. 

First Series. Production of a Current oy a Current. 



281. Let there be two wires, covered with silk, so arranged 
about a common support as to form two helixes in a manner 
parallel to each other through their whole extent. This being 
supposed, if we connect the two extremities of one of the wires 
with the poles of a powerful voltaic apparatus, and connect the 
extremities of the other wire with a galvanometer, we remark, 
1. that a current takes place in the wire of the galvanometer in a 
direction opposite to that of the generating current ; 2. that the 
current ceases forthwith, since the galvanometer resumes its first 
position ; 3. that a current in a reverse direction is manifested in 
the galvanometer, at the moment the communication is destroyed 
between the generating wire and the voltaic apparatus. 

Several other experiments are suggested by the above facts. 
If we substitute a helix for the galvanometer, and place a steel 
wire in it, it will be magnetized in one direction the moment the 
communication is established ; but there will be no magnetism 
communicated, if we place the steel wire and withdraw it during 
the continuance of the generating current ; lastly, it is magnetized 
in a reverse manner, if we introduce the steel wire while the gen- 
erating current is in action, and allow it to remain till the moment 
of interruption. 



Of Magnetizing by Electric Currents. 337 

Second Series. 

282. If a wire connected with a galvanometer is brought near 
to a wire communicating with a voltaic apparatus in action, there 
is produced in the former a current in a direction opposite to that 
of the latter ; whereas the current takes place in the same direc- 
tion, if we withdraw the first wire from the second* If the first 
wire is kept in the same position, no current is manifested* 

The substitution of a magnet for the generating current of the 
Voltaic apparatus, gives rise to phenomena analogous to the pre- 
ceding. If, for example, we surround the armature, consisting 
of soft iron, of a powerful horse-shoe magnet with a copper wire 
covered with silk, and communicating with a galvanometer, we 
find, that the application of the armature to the magnet causes a 
deviation in the galvanometer, in a direction opposite to that pro- 
duced by the current which puts the armature in the state to 
which it is brought by the magnet. If we withdraw the armature 
from the magnet a deviation in a contrary direction is manifested* 
During the contact the galvanometer is not affected* 

Magnets ought evidently, according to this last fact, to act 
upon helixes. Indeed, if we introduce a magnet into the interior 
of a helix, whose two extremities communicate with a galvanome- 
ter, it will be seen, on approaching it suddenly, that a. deviation 
takes place in the galvanometer. If we withdraw it, a deviation 
in a contrary direction is observed. If we bring the magnet to 
the middle of the helix, there is no deviation. Lastly, the galva- 
nometer exhibits a deviation the reverse of the first, when the 
magnet is withdrawn at the extremity opposite to that at which it 
is introduced. 

In all these cases, the galvanometer does not indicate a current 
when the magnet is at rest. It may be supposed, that*, if we were 
to place a piece of soft iron in the helix, the effects manifested 
Upon the approach of a magnet and its removal would be increas- 
ed ; this is, in fact, what is found to take place. 

283. A great number of philosophers have sought in vain to 
produce sparks by the action of magnets. Dr. Faraday's experi- 
ments upon this subject have at length been crowned with success. 
The problem may indeed be considered as solved by the forego- 
ing experiments, since magnets are found to produce currents. 

E. Sf M. 43 



338 Electro-Dynamics, 

The following simple apparatus is fitted to exhibit all the known 
effects of the voltaic current. 
Fig. 179. MNP is a powerful magnet, movable about a vertical axis 
PQ. The poles M, N, of this magnet are made to pass succes- 
sively near the extremities A, B. If we call to mind the facts 
above stated, it will be seen, that there must take place, at each 
passage of the magnet, below the armature of soft iron ACB, two 
opposite currents. 

The wire VR may be made use of to verify all the effects of 
voltaic currents ; as the production of sparks, the charge of con- 
densing instruments, the shock, and, lastly, the decomposition of 
water. The current changing with each semi-revolution of the 
magnet, the two constituents of water disengage themselves suc- 
cessively at the extremity of the same wire. By means of a see- 
saw, properly arranged, each gas may be obtained separately. 



Description of an Apparatus by Means of which all the Experi- 
ments relating to Electro-Dynamics may be performed. 

284. To unite the advantages of particular instruments, it is 
necessary to render permanent the parts of the apparatus adapted 
to the operations which are common to all the experiments, and 
then to put successively in communication with the common parts 
the several movable conductors, each of which is attached separ- 
ately. Such was the object which M. Ampere proposed, in the 
instrument of which the following is a description. 

In the experiments on dynamic electricity, a portion of the vol- 
taic circuit, rendered movable, is subjected to the action of a 
fixed conductor, of a magnet, or of the earth. In order that the 
circuit may not be interrupted, this movable part must be con- 
nected with the system of conductors by cups filled with mercury, 
a metal made use of to unite the parts of the apparatus which we 
Fig. 180. cannot always fasten or solder. The table g h, which supports 
the apparatus, should be covered with an insulating varnish, and 
the greatest care should be taken to prevent its being wet with any 
acid solution, and especially to wipe off every particle of mercury 
that may have fallen upon it in the course of the experiments. 
An opening for this purpose is made in the middle of the table, 
and communicates with a drawer underneath. 



Description of an Apparatus. 339 

It is important, in all our experiments with this instrument, that 
we should interrupt the electrical current in the movable conduc- 
tors, whenever we would introduce the points of these conductors 
into the cups or withdraw them. This precaution is necessary in 
order to avoid a combustion or fusion of the points. It is proper, 
also, before beginning these experiments, to satisfy ourselves, that 
the current passes completely through both the fixed and the 
movable conductor. This is easily done by means of a galva- 
nometer, 

As to the suspensions, they are effected by means of small cups 
filled with mercury, arranged as the case may require. The two 
wires of the voltaic apparatus are fixed to the table by means of a 
thumb-screw, during the course of the experiments, so that they 
may be introduced at pleasure into the grooves A, a. The ex- 
perimenter is able, by a simple operation, to suspend the electro- 
dynamic action, or to change the direction, by reversing the course 
of the current in both the fixed and the movable conductors. 

Suppose the experimenter facing the table g h, having on his 
right hand the wires R, r, and before him on the same side the 
two see-saws K, k, fitted to change the direction of the current 
in the fixed and movable conductors. These see-saws, raised 
about an inch above the table, consist each of two copper plates 
separated by a piece of ivory or varnished wood. On the lateral 
edges of the two plates are eight projections, four of which, situat- 
ed on the right, when we incline this way the see-saw nearest the 
operator, pass into the grooves A, B, and into the cavities C, -D, 
(fig. 181.) The four other projections, when the see-saw is in- 
clined to the left, dip into the same grooves A, B, and into the 
cavities C", -D\ But, by preserving the see-saw in a horizontal 
position, the communication between A and B and the cavities 
C, D, and C, D', is interrupted. These cavities communicate, 
two and two, that is, C with O, and D with D 1 , by means of 
upper strips placed crosswise under the table, covered with silk, 
and separated from each other by a piece of varnished wood. The 
cavities G, H, (fig. 180,) answer to strips of copper, which termi- 
nate, one in the cavity C, and the other in the cavity If. These 
are, therefore, also in communication, the first with C, and the 
second with D. 

The see-saw k serves, according to the direction in which it is 
inclined, to establish or interrupt the communication between the 



340 Electro-Dynamics. 

grooves B, a, and the cavities c, d, and c', d'. These, united in 
couples, by a contrivance similar to the preceding, are so arranged 
that c, c', communicate, by means of a strip of copper cfs and 
the spring IF, placed under the table, with the cup 8, the height of 
which is adjusted by means of the screw z, placed under the ta- 
ble ; they communicate also with the column f u. By the side of 
these, dand d! communicate, first with the two semicircular grooves 
MN, m n, containing mercury, in which are introduced the two 
extremities of the wire t u v, of a compass needle, and subsequent- 
ly with the column FU, and with the cavity O. 

The columns/ u, FU, are of copper, and serve, according to 
the position of the see-saw k, the one to carry the current to the 
movable conductors, and the other to bring it back.* 

For this purpose, the column FU communicates with the cup 
X, and fu with the cup Y, insulated by a glass tube covered 
with gum-lac. X communicates with x, x', and Y with y, y' . 
Each system of two cups, x, y, and x' , y', furnishes to the mov- 
able conductors a vertical suspension, passing through the centre of 
the cups ; while x with %j and x' with y present a horizontal sus- 
pension, susceptible, by means of the knob Z, of being put in any 
azimuth. 

It will be readily seen, that, when we introduce the wires from 
the voltaic apparatus into the grooves A, a, the current cannot 
take place through the columns/ u, FU, because, by following the 
series of metallic pieces in connexion with the two grooves, we 
find, that it is interrupted in two places, first, between the cups 
above mentioned, secondly, between the two cavities G, H. made 
in the table and filled with mercury. By placing either one of 
the movable conductors, represented in figures 172, 173, in the 

* In the first instruments constructed by M. Ampere, it was the col- 
umn ET, which here only serves as a support to the cross-piece UT, 
which was employed to establish the communication. According to 
256. the doctrine above laid down, the communication should be made by 
the column fu. The former construction was faulty, since the action 
of the portion of the current established in ET would conspire with 
a portion of the current in the column FU, to alter the motion of the 
movable conductors ; whereas, on the improved form of the instru- 
ment, the action of the current/?/, destroys that of FU, so much the 
more completely as the columns are near together. It is easy to cor- 
rect this defect in the apparatus, where it exists. 



Description of an Apparatus. 341 

cups x, y, or #', y', or the conductor represented in figure 167, in 
the cup x, y' or x', y, we establish the communication between the 
two columns, and the voltaic circuit is interrupted only between 
the two cavities GH. If, therefore, we would observe the action 
of the earth upon the conductors represented in figures 169, 166, 
the only ones upon which it can act, since the others are astatic, 
it is necessary, after having placed them in the cups, to establish 
the communication between the cavities G, H, (fig. 180,) which is 
done by means of a piece of copper Q q, turning about the hinge 
q, attached to the table; and the projections e,f are so arranged, 
when it is reversed, as to enter the cavities G, H ; when these 
points are in contact with the mercury the circuit becomes closed, 
the current is established, and the action of the earth upon the 
movable conductors manifests itself forthwith. 

But, if we would observe the action of a current upon the differ^ 
ent movable conductors, we restore the piece Q q to the situation 
represented in figure 180 ; the current is again interrupted, but 
is reestablished by placing in the cavities G, JET, the extremities, 
marked by the same letters of the fixed conductors which are 
represented in figures J 82, 183. The first of these is composed 
of a horizontal wooden base RS, furnished below with four points 
L, L, L, L, which are placed in four holes made in the table 
(fig. 180), designated by the same letters. In the middle of this 
base is raised an upright piece KL, which is also made of wood 
and supports the table P, Q. Upon the base is arranged a copper 
wire with two branches, whose extremities are marked G, H, G', 
H'. When the first of these are placed in the cavities G, H, (fig. 
180,) it is the part p o of the wire, situated on the right, which 
acts on the part b c of the astatic movable conductor (fig. 173) 
suspended from the cups x, y (fig. 180), to attract or to repel it 
according as the electric current proceeds in the same or in oppo- 
site directions, along the two vertical portions of the conducting 
wire. 

We also place in these cups the extremities x, y, of the mov- 
able conductors, represented in figures 184, 185, for the purpose 
of showing, that the current of the wire o p (fig. 183) has no ac- 
tion upon these conductors. 

When the extremities G', H', (fig. 183) are introduced into 
the cavities G, H, (fig. 180,) the portion b c of the movable 
conductor (fig. 173) is situated between the two portions of the 



342 Electro- Dynamics. 

conducting wire m n, and v t of the fixed conductor (fig. 183), of 
which one is straight and the other sinuous, so that they both at- 
tract or both repel, with equal forces, the current of the wire b c, 
(fig. 173.) 

The other fixed conductor (fig. 1S2) has a base RS, similar 
to that of the preceding, which supports a vertical frame of wood 
PMNO, around which is bent several times, for the purpose of 
increasing the action, a copper plate, covered with silk, the ex- 
tremities of which are seen at G and H. These extremities are in- 
troduced into the mercury of the cavities G, H, (fig. 180,) 
whether the points L, L, L, L, (fig. 182,) of the base RS are 
placed in the holes L, L, L, L, (fig. 180,) or in the holes U, LI, 
U, U. In the first case, the frame PMNO is placed vertically 
below the rod TU (fig. 180). In the second case, it is placed 
out of a perpendicular, by a quantity equal to the radii of the arcs 
of the movable conductor (fig. 175), and of the vessel (fig, 
178). 

It is in the first of these two positions that we place the fixed 
conductor, when we would make use of it, instead of the precede 
ing, for the experiment upon the mutual action of two parallel 
currents. In this case, the movable conductor of figure 167 is to 
be employed. The counterpoise i is used, to preserve the mov- 
able conductor in equilibrium by the side of, and at a small dis- 
tance from, the fixed conductor. 

It is also with the conductor (fig. 182), that we perform all the 
experiments relative to the action of two angular currents about to 
be described. 



Experiments upon the Mutual Action of two Rectilinear Currents, 
making any Angle with each Other. 

285. If, after having placed the astatic conductor (fig. 172) in 
the situation in which it corresponds vertically with the rod TU, 
we put the two points x, y, of the movable conductor into the 
cups x, y, (fig. 180,) the point e being very near the point N 
(fig. 182), the rectilinear currents d e (fig. 172) and MN (fig. 
1S2) making any angle with each other, we determine, by the 
motion of the movable conductor, the attractive or repulsive ac^ 
tion thus exerted. 



Mutual Action of two Rectilinear Currents. 343 

When the same movable conductor is suspended from the 
cups x', y 1 (fig. 180), the point e corresponding to the middle of 
the part MN of the rectangle (fig. 182), an action now takes 
place in two angles at once. In one it is attractive, in the other 
repulsive. The effect produced is the same as in the preceding 
arrangement; but the intensity is doubled. The branch p serves 
only as a counterpoise. 

The movable conductor (fig. 173) may be placed alternately 
in the cups x, y, and x', y' ; in the first case the current coming al- 
ways through the cups X, x, takes the direction x a b c d efg h i 
y. Then in the wire c d, g A, this current is still directed toward 
the vertex of the angle which these two wires make with the fixed 
conductor ; hence, if we place the movable conductor in a plane 
perpendicular to that of the fixed rectangle, so that the points e, h, 
shall be very near the extremity N (fig. 182J, the two portions 
c d, g h, will be both attracted or both repelled, according to the 
direction of the current of the fixed conductor. In the case of 
attraction, there will result an unstable equilibrium ; for, if one 
of these wires is accidentally a little nearer to the fixed conductor 
than the other, it will be more strongly attracted and carried to- 
ward it. If we change the direction of the current in either con- 
ductor, attraction will be changed to repulsion, and the movable 
conductor will place itself perpendicularly to the fixed conductor. 
In this case the equilibrium will be stable. 

When we suspend the same conductor from the cups x', y', the 
points d, h, correspond to the middle of MN, and, as the current 
takes a direction opposite to that of the preceding, and passes 
through the movable conductor, in the direction yihgfedcb 
a x, it separates, in the wires c d and g h, from the middle part 
d h, so that these two wires are carried with equal forces in the 258. 
direction of the current of the rectangle which tends to incline 
the movable conductor a little in this direction, but does not im- 
press upon it any motion of rotation. 

The conductor (fig. 174), suspended from the cups x', y', al- 
lows a passage to the electric fluid in the direction x' a b c d efg 
h i y'. If, therefore, it first intersects the fixed conductor MN 
(fig. 182) at any angle, and the current is directed the same way 
in each, it will tend to move until the branch d e is parallel to M 
N. Moreover, the movable conductor will be seen to make a 
semi-revolution, if we change the direction of one of the currents. 



344 Electro-Dynamics. 

The apparatus represented in figure 175 is intended to show, 
that, in the conductor designated in figure 174, the branches c d, 
ef, contribute to the effect produced ; for it is only the wires de- 
signated by the same letters in figure 175, which, being capable of 
receiving motion from the fixed conductor, contribute to the mo- 
tion of the movable conductor. The actions of this conductor 
upon each of the elements of the horizontal circles of the movable 
conductor, passing through the axis of rotation, can contribute 
nothing to the motion of this last. We see, nevertheless, that a 
motion takes place, with a force indeed much less. The same 
movable conductor (fig. 175) serves, beside, as well as that of 
figure 142, for experiments relative to the mutual action of two 
currents, whose directions are constantly at right angles with each 
other* 



Experiments on Continued Rotation produced by a fixed Con- 
ductor, by the Action of the Earth, or by that of Currents, 
which are established in the acid Solution, in which the mov- 
able Conductors are immersed. 

286. In all the experiments upon continued rotation, we make 
use of the tripod (fig. 17S), placing the three points O, /, V in the 
cavity and holes designated by the same letters in the table (fig. 
180). We then put upon this tripod the copper vessel (fig. 186), 
the foot of which, Jl, enters the cavity/, (fig. 178,) communicating, 
by the copper plate 10, with the cavity O (fig. 180). This ves- 
sel is filled with an acid solution, in which is immersed the inferior 
part of all the movable conductors employed in experiments of 
this kind. The current no longer passes into the columns/ u, 
FU, which in this case can be of no use, since the movable con- 
ductors are removed ; but, after having passed the fixed conduc- 
tor, it arrives in the groove B, enters the cavity c', the see-saw k 
being inclined to the right, reaches the cup S, and passes over the 
movable conductor, which is suspended in it, traverses the acid 
solution of the vessel, the vessel itself, and arrives at the resinous 
wire, through O o c. In the apparatus just described, the motion 
is produced by the action of a spiral, placed on the edge of the 
tripod, so as to surround the vessel. Along the two other feet of 
the tripod, are made to descend the two extremities of the copper 



Continued Rotation produced by a fixed Conductor. 345 

plate which forms the spiral ; one being made to enter G and the 
other H. When the motion is to be produced by a rectilinear 
current, tangent to the vessel, we employ the rectangle (fig. 182), 
and we take care to remove the extremities G and H (fig. 178) 
of the spiral, so that they no longer enter the corresponding cavi- 
ties G, H, (fig. 180.) In this case it is the projections G, H, of 
the rectangle (fig. 182), which are to be immersed, by turning this 
rectangle about and placing the points L, L, L, L, in the holes 
L', U, U, U, (fig. 180.) 

We have already considered the conductors represented in fig- 
ures 142, 168, 187, 138, which are employed in these experi- 
ments. 

By making the spiral current (fig. 178) act upon the two in- 
struments (figs. 142, 168), they will both turn with a continued 
rotation, the velocity of which, at first accelerated, soon becomes 
constant ; but, by subjecting the two conductors to the action of 
the current of the rectangle (fig. 182), the instrument (fig. 168) 
will still turn with a continued rotation, the velocity of which nev- 
er becomes uniform, but experiences alternate variations, accord- 
ing as the radius a c is, at each revolution, sometimes nearer to, 
sometimes farther from the rectangle. As to the instrument (fig. 
142), it will no longer tend to turn except by the action of the 
earth, and that of the rectangle (fig. 182) will tend to bring it 
into a fixed position, in which the plane abed will be always par- 
allel to the plane of this rectangle, so that the branch a b shall be 
on the side whence comes the current established in MM, when 
that of a b is descending, and on the opposite side, when that of 
a b is ascending. Lastly, by subjecting to the sole action of the 
earth the instrument represented in figure 168, it will be seen to 
turn with a constant velocity ; this is not the case with the instru- 
ment represented in figure 142, since, beside the action which the 
earth exerts upon the branch b s, in producing a uniform motion, 
it acts also upon the vertical branch a b, bringing it back to a 
fixed position, to the east when the current is descending, and to 
the west when it is ascending. 

It is important to remark, that, when we would admit the action 
of the earth, or of the currents of acid solution, we must establish 
a communication with the grooves JIB (fig. 180), by the conduc- 
tor g-Q; then the current passes the movable conductors, either 
by traversing the two columns f u, FU, or by arriving in the cup 

E. fy M. 44 



346 Electro- Dynamics, 

S, according as the movable conductors are suspended from the 
cups x y, x' 2/, or from the cup S. 



Experiments upon Helixes and Solenoids. 

287. The figures 188, 171, represent conductors in the form of 
a helix, with which we are able to imitate magnets. The first 
is fixed to the table (fig. 180) by means of the screw b, so that 
the two projections G, H, enter the cavities designated by the 
same letters. The second (fig. 171) is suspended from the cups 
x y, or x' y' ; the current is established, therefore, in the former 
as it would be in another fixed conductor, and in the latter as it 
would be in another movable conductor ; upon these helixes are 
verified the propositions already demonstrated. 

Of the Multiplier. 

288. M. Schweiger, of Halle, invented an apparatus for the 
purpose of showing the feeblest electrical currents. The con- 
struction of this instrument depends upon the equal action of all 
parts of a conducting wire, and upon the circumstance, that a wire, 
bent round so as to return upon itself once, produces a double 
effect (fig. 189), and, indeed, that the effect is proportional to the 
number of circuits made by the wire. According to this princi- 
ple, the power of the instrument may be augmented indefinitely. 

Figure 190 represents this instrument, in which J1A is the base, 
CC, CO, two upright pieces, supporting the frame BB, on the 
outside of which is a groove, that receives the successive coils of 
the multiplying wire. DD is an upright stem, destined to support 
the wire from which the magnet is to be suspended ; all these parts 
are of wood. EE is a metal rod, which fits close into an opening 
made in the support DD ; to this rod is attached, by a little wax, 
a fibre of silk EF, which carries at its extremity a magnetic nee- 
dle. The suspension wire passes through a cylinder, which pre- 
vents the multiplying wire from touching it. 

There is, moreover, below the magnetic needle a graduatea 
circle, U, which measures the deviations. The multiplying wire 
is of copper or silver, and about a hundredth of an inch in diam- 



Of the Multiplier. 347 

eter. It is covered with silk through its whole extent, which pre- 
vents all communication between the different parts of the wire, 
that lie upon each other in the groove of the frame BB. H, J, 
represent the two extremities of the wire. 

The use of this instrument will be readily understood. In or- 
der to multiply the action produced on the needle by the voltaic 
current, we have only to establish the communication in such a 
manner, that the multiplying wire shall become a part of the cir- 
cuit. The directive force of the earth tends always to bring the 
needle into the magnetic meridian. Accordingly, if we would 
give to the instrument all the sensibility of which it is susceptible, 
we must diminish this force without entirely destroying it ; since, 
in this case, the most feeble currents would have the same effect 
upon the position of the needle as the most powerful. We reduce 
the directive force of the earth by suspending two needles, with 
the poles of the one opposed to those of the other, the one being 
in the circuit of the wires, and the other without. This instru- 
ment is of great value at the present time. 



NOTES. 
I. 

The Torsion Balance. 



After a very careful analysis of the effect of torsion in the case 
of wires, Coulomb made a very happy application of this principle 
to the construction of an instrument for the purpose of measuring 
all kind of small forces. This instrument consists principally of a 
vertical wire, the upper end of which is attached to a fixed point, 
and the lower end, kept steady by a small weight, carries a horizon- 
tal needle. When we would estimate very small forces, we bring 
them to act upon the extremity of this needle, and measure their 
intensity by the angle through which they cause it to diverge from 
its point at rest. In a word, we balance the force in question by the 
torsion of the wire ; and it is for this reason, that Coulomb has given 
to this instrument the name of torsion balance. 

To prevent the needle from being agitated by the air, it is en- 
closed in a cylindrical glass case ; and the wire is likewise enclosed 
in a hollow glass cylinder, at the top of which is placed a graduated 
circle, which turns with considerable friction about the cylinder. 
The stem to which the wire is attached, carries a horizontal index, 
which moves over this circle, and serves to point out the number 
of degrees, when we wish to give the wire a determinate torsion. 
There is likewise a circular division applied horizontally about the 
glass case to measure the range of the needle. 

We give the wire and needle different lengths and magnitudes, 
depending upon the object we have in view. If the forces we wish 
to measure are very small, in which case the instrument must have 
great sensibility, we use long and fine wires ; for the force of tor- 
sion is inversely as the length of the wire, and directly as the fourth 
power of its thickness. Long wires have this advantage also, that 
we can twist them through a greater number of degrees without 
changing the law of their elasticity. It is necessary, moreover, to 
use those substances whose elasticity is most perfect. 



350 Notes. 

The torsion balance will serve to render sensible the universal 
attraction, which takes place between all bodies in nature, in the 
direct ratio of their masses, and in the inverse ratio of the squares 
of their distances, and by virtue of which, under the name of gravi- 
ty, all bodies around us tend towards the centre of the earth. Sup- 
pose the needle to be at rest, in a position determined by the natural 
state of the wire, and that two spheres, of any substance whatever, 
are brought toward the extremities of the needle on opposite sides. 
If they really exert an attraction at a distance upon the particles of 
the needle thus suspended, and if they are attracted in turn by the- 
needle, the needle must be moved from its original position ; and its 
extremities must approach the spheres which attract it, until the 
force of torsion of the wire, which opposes this motion, is sufficient 
to counterbalance the attraction. The needle will not stop, how- 
ever, at the precise instant when this equilibrium takes place, but 
will continue to move, not in virtue of the attraction, but in conse- 
quence of its velocity previously acquired. It will, therefore, ad- 
vance until the force of torsion, always increasing, destroys this 
velocity and begins to bring the needle back to its position of rest ; 
it then passes this point to a certain distance on the other side, after 
which it begins again to move towards the spheres ; and thus it will 
perform a series of oscillations. The effect may be rendered more 
sensible by giving the needle such a form, that the greater part of 
its mass shall be situated towards its extremities ; which will be the 
case, if we use a cylindrical needle, terminated at each end by a ball 
of a considerable diameter compared with that of the cylinder. This 
will have the additional advantage of facilitating the calculation ; 
for among the laws of attraction it is shown, that a homogeneous 
sphere acts upon a point situated without i^ as if its whole mass 
were united in a single point at its centre ; and, although the mass 
of the needle can never be rendered absolutely nothing, yet it is 
manifest, that, if it be very small compared with the mass of the 
spheres which terminate it, its influence must be proportionally 
feeble, and may be easily allowed for. We are able then to obtain, 
by the laws of mechanics, an expression for the forces which attract 
the two spheres, when they oscillate with the observed velocity, in 
the presence of attracting bodies, which may also, for the sake of 
simplicity, be considered as spherical. If we compare the duration 
of these oscillations with the durations of the oscillations of a verti- 
cal pendulum, produced by the action of the terrestrial globe, we 
shall have the ratio between this force and that of the spheres in 
question ; hence, we deduce the ratios between the masses of these 
bodies and the mass of the earth ; and, as the bulks of these bodies 



Notes. 351 

are supposed to bo known, we shall obtain the ratios of their densi- 
ties. Cavendish, who made this fine experiment, found, in the way 
we have stated, the mean density of the earth, that of water being 
unity, to be equal to 5,5. 

Coulomb applied the torsion balance to the purpose of measuring 
the intensities of electric and magnetic forces. He even used it to 
ascertain the adhesive force of liquids, considered with respect to 
themselves and to other bodies. For this purpose, he immersed in 
the liquids, plane discs, suspended by their centres in a horizontal 
position by means of wires of a known force, and he compared to- 
gether the velocities of the oscillations performed by these discs in 
the liquids and in the air. 



II. 



Instructions respecting the best Form $*c. of Lightning Rods, extract- 
ed from a Memoir of M. Gay-Lussac. [Annales de Chimie.] 

The most advantageous form that can be given to lightning-rods, 
appears evidently to be that of a very sharp cone ; and the higher 
it is elevated in the air, other circumstances being the same, the 
more its efficacy will be increased, as is clearly proved by the exper- 
iments with electrical kites, made by MM. de Romas and Charles. 

It has not been accurately ascertained, how far the sphere of ac- 
tion of a lightning-rod extends ; but, in several instances, the more 
remote parts of Targe buildings on which they have been erected, 
have been struck by lightning at the distance of three or four times 
the length of the conductor from the rod. It is calculated by M. 
Charles, that a lightning-rod will effectually protect a circular space, 
whose radius is twice the height of the conductor ; and they are now 
attached to buildings according to this principle. 

A current of electric matter, whether luminous or not, is always 
accompanied by heat, the intensity of which depends on the velocity 
of the current. This heat is sufficient to make a wire red-hot, or 
to fuse or disperse it, if sufficiently slender ; but it scarcely raises 
the temperature of a bar of metal, on account of its large mass. 
It is by the heat of the electric current, as well as by that disengaged 
from the air, condensed by the passage of the lightning through it, 
when not conveyed by a good conductor, that buildings struck by it 
arc frequently set on fire. 



352 Notes. 

No instance has yet occurred of an iron bar, of rather more than 
half an inch square, or of a cylinder of the same diameter, having 
been fused, or even heated red-hot by lightning. A bar of this size 
would therefore be sufficient for a lightning-rod ; but, as its stem ought 
to rise from 15 to 20 feet above the building, it would not be strong 
enough to resist the action of the wind, unless the lower part were 
made much thicker. 

An iron bar, about three-quarters of an inch square, is sufficient 
for conductors. It might even be made still smaller, and consist 
merely of a wire, provided it be connected at the surface of the 
ground with a bar of metal, about half an inch square, immersed in 
water, or a moist soil. The wire, indeed, would pretty certainly be 
dispersed by the lightning, but it would direct it to the ground, and 
protect the surrounding objects from the stroke. However, it is 
always better to make the conductor so large as not to be destroyed 
by the stroke ; and the only motive for substituting a wire for a stout 
bar is the saving in point of expense. 

The noise of the thunder generally occasions much alarm, although 
the danger is then passed ; it is over, indeed, on the appearance of 
the lightning, for any one struck by it neither sees the flash, nor 
hears the report. The noise is never heard till after the flash, and 
its distance may be estimated at so many times ] 136 feet as there 
are seconds between the appearance of the lightning and the sound 
of the thunder. 

Lightning often strikes solitary trees ; because, rising to a great 
height, and burying their roots deep in the soil, they are true light- 
ning-rods, and they are often fatal to the individuals who seek them 
for shelter ; since they do not convey the lightning with sufficient 
rapidity to the ground, and are worse conductors than men and ani- 
mals. When the lightning reaches the foot of the tree, it divides 
itself amongst the neighbouring conductors, or strikes some in 
preference to others, according to circumstances; and sometimes 
it has been known to kill every animal that had sought shelter under 
the tree ; at others, only a single one out of many has perished by 
the stroke. 

A lightning-rod, on the contrary, well connected with the ground, 
is a certain security against the effects of lightning, which will never 
leave it to strike a person at its foot ; though it would not be pru- 
dent to station one's self close to it, for fear of some accidental break 
in the conductor, or of its not being in perfect communication with 
the ground. 

When lightning strikes a house, it usually falls on the chimneys, 
either from their being the most elevated parts, or because they are 



Notes. 353 

lined with soot, which is a better conductor than dry wood, stone, 
or brick. The neighbourhood of the fireplace is consequently the 
most insecure spot in a room during a thunder-storm. It is best to 
station one's self in a corner opposite the windows, at a distance from 
every article of iron or other metal of any considerable size. 

Persons are often struck by lightning without being killed ; and 
others have been wholly saved from injury by silk dresses, which 
serve to insulate the body, and prevent the access of the electric 
matter. 

The stem, or part of the rod above the building, should be a 
square bar of iron, tapering from its base to the summit, in the form 
of a pyramid. For a height of from 20 to 30 feet, which is the mean 
length of the stems placed on large buildings, the base should be 
about 2£ inches square. 

Iron being exposed to rust by the action of the air and moisture, 
the point of the stem is liable to become blunt ; to prevent this, a 
portion is cut off from the upper end, about 20 inches in length, and 
replaced by a conical stem of brass or copper, gilt at its extremity, 
or terminated by a small platina needle, two inches long.* The 
platina needle should be soldered with silver solder to the copper 
stem; and to prevent its separating from it, which might sometimes 
happen, notwithstanding the solder, it is secured by a small collar 
of copper. The copper stem is united to the iron one by means of a 
gudgeon which screws ir/to each. If the gilding of the point cannot 
easily be performed on the spot, nor the platina readily obtained, 
they may both be dispensed with without any inconvenience, and 
a plain conical copper stem only be employed. Copper does not rust 
to any considerable depth in the air, and even if the point becomes 
somewhat blunt, the rod will not thereby lose its efficacy. 

Below the stem, three inches from the roof, is a cap, soldered to 
the body of the stem, and intended to throw off the rain water, which 
would flow down the stem, and tend to injure the building, 

Immediately above the cap, the stem is rounded for about two 
inches to receive a split collar, with a hinge and two ears, between 
which the extremity of the conductor of the lightning-rod is fixed 
by a bolt. Instead of the collar, we may make use of a square 
stirrup, embracing the stem closely. The stem of the lightning-rod 
is fixed on the roof of buildings, according to circumstances. If it 
is to be placed above a rafter, the ridge must be pierced with a 
hole through which the foot of the stem passes, and is steadily fixed 

# Instead of a platina needle, one of standard silver may be substituted, 
composed of nine parts of silver, and one of copper^ 
E. fy M. 45 



354 - Notes. 

against the king-post by means of several clamps. This disposition 
is very firm, and should be preferred if the circumstances admit of it. 

If the stem be fixed on the ridge, a square hole must be made 
through it of the same dimensions as the foot of the stem ; and above 
and below we fix, by means of bolts, or two bolted stirrups, which 
embrace and draw the ridge together, two iron plates about three- 
quarters of an inch thick, each having a hole corresponding to that 
in the woodwork. The stem rests by a small collet on the upper 
plate, against which it is strongly pressed by a nut, made to screw 
on the end of the stem against the lower plate. 

Lastly, if the lightning-rod is to be fixed on a vaulted roof, it should 
be terminated by three or four feet, or spurs, which must be solder- 
ed into the stone, with lead, in the usual manner. 

The lower part of the conductor should be an iron bar or rod 
about three-quarters of an inch thick, reaching from the bottom of 
the stem to the ground. It should be firmly united to the stem by 
means of a collar, screw, or bolt, and its several parts should be 
connected together in a similar manner. After penetrating into the 
ground for about two feet, it should be bent at right angles to the 
wall of the building, and, after being carried in that direction for 
twelve or fifteen feet, it should be made to communicate with a 
well, drain, aqueduct, or permanently moist earth. If the soil be 
dry, it should extend to the depth of twelve or fifteen feet; and, to 
secure it from rust, it should be surrounded with charcoal, which is 
indestructible, and which, while it preserves the iron, facilitates the 
passage of the electricity into the ground by its conducting property. 

Both the bottom and top of a lightning-rod are sometimes made to 
terminate in several branches, and its efficacy is thus increased. It 
is also recommended to connect with the lightning-rod any large 
masses of iron that may be in the building, as metal pipes, and gut- 
ters, iron braces, &c. ; without this precaution the lightning might 
strike from the lightning-rod to the metal, especially if there hap- 
pened to be any interruptions in the former, and thus occasion seri- 
ous injury to the building, and danger to its inhabitants. 

In the case of powder magazines, the lightning-rod should not be 
attached to the building, but to poles eight or ten feet from it. If 
the building be large, several should be used, arranged according 
to the rule, t1!at a lightning-rod may be considered as protecting a cir- 
cular space ivhosc radius is twice the height of the rod. If the maga- 
zine be in a tower or other very lofty building, it may be sufficient to 
defend it by a double copper conductor, without any stem. As the 
influence of this conductor will not extend beyond the building, it 



Notes. 355 

cannot attract the lightning from a distance, and yet it will protect 
the magazine, should the lightning happen to fall upon it. 

In the case of a vessel, the stem may consist merely of the copper 
point, already described. It should be screwed on an iron rod, rising 
above the top-gallant mast, and connected, by means of a hook or 
ring at its other extremity, with a metallic rope extending to the 
water or copper sheathing of the vessel. Large ships should be 
provided with two conductors, one on the main mast, and one on the 
mizen mast. 

The experience of fifty years demonstrates, that, when constructed 
with the requisite care, lightning-rods effectually secure the buildings 
on which they are placed, from being injured by lightning. In the 
United States, where thunder-storms are more frequent and more 
formidable than they are in Europe, their use is become general; a 
great number of buildings have been struck, and scarcely two are 
quoted as not having been saved from danger. The apprehension 
of the more frequent fall of lightning on buildings provided with light- 
ning-rods, is unfounded, for their influence extends to too small a 
distance to justify the idea, that they determine the lightning of an 
electric cloud to discharge itself on the spot where they are erected. 
On the contrary, it appears certain, from observation, that buildings 
furnjshed with lightning-rods are not more frequently struck than 
formerly. Besides, the property of a lightning-rod to attract the 
lightning must also imply that of transmitting it freely to the ground, 
and hence no danger can arise as to the safety of the building. 

We have recommended the use of sharp points for lightning-rods, 
as having an advantage over bars rounded at the extremity, by con- 
tinually pouring off into the air, whilst under the influence of a 
thunder-cloud, a current of electric matter in a contrary state to that 
of the cloud, which must probably have some effect towards neutral- 
izing the state of the latter. This advantage must by no means be 
neglected ; for it is sufficient to know the power of points, and the 
experiments of M. Charles and M. Romas with a kite flown under 
a thunder-cloud, to be convinced, that, if sharp-pointed lightning-rods 
were placed in considerable numbers on lofty places, they would 
actually diminish the electric matter of the clouds, and the fre- 
quency of the fall of lightning on the surface of the earth. How- 
ever, if the point of a conductor should be blunted by lightning, 
or any other cause, we are not to suppose, because it has lost the 
property we have mentioned, that it has also become ineffectual to 
protect the building. Dr. Rittenhouse relates, that having often 
examined the extremities of the lightning-rods in Philadelphia, where 
they are very general, with an excellent telescope, he observed 



356 Notes, 

many whose points had been fused ; but he never found that the 
houses on which they were erected had in consequence been struck 
by lightning. 

[The original memoir of M. Gay-Lussac not being at hand, the 
above was extracted, with a few alterations, from a translation con- 
tained in the Annals of Philosophy.'] 



in. 

Hare's Calorimotor and Deflagrator. 

1. Calorimotor. 

This name is given by Dr. Hare to an instrument invented by him, 
in which the calorific effects are accompanied with a feeble influ- 
ence upon the electroscope. 
Fig. 74. "A, a, represent two cubical vessels twenty inches square, 
b b b b, a frame of wood containing twenty sheets of copper, and 
twenty sheets of zinc, alternating with each other, and about half 
an inch apart, TT t t, masses of tin, cast over the protruding edges 
of the sheets, which are to communicate with each other. Fig. 74' 
represents the mode in which the junction between the several 
sheets and the tin masses is effected. Between the letters z z, the 
zinc only is in contact with the tin masses. Between c c, the cop- 
per alone touches. It may be observed, that, at the back of the 
frame, ten sheets of copper between c c, and ten sheets of zinc be- 
tween z z, are made to communicate by a common mass of tin ex- 
tending the whole length of the frame between TT; but in front, 
as shown in figure 74, there is an interstice between the mass of tin, 
connecting the ten copper sheets, and that connecting the ten zinc 
sheets. The screw forceps, appertaining to each of the ten masses, 
may be seen on either side of the interstice; and likewise a wire 
for ignition held between them. The application of the rope, pul- 
ley, and weights is obvious. The swivel at S, permits the frame to 
be swung round and lowered into water in the vessel a, to wash off 
the acid, which, after immersion in the other vessel, might continue 
to act on the sheets, encrusting them with oxide. Between p p, 
there is a wooden partition, which is not necessary, though it may 
be beneficial." 

" Volta considered all galvanic apparatus," says Dr. Hare, " as 
consisting of one or more electromotors, or movers of the electric 
fluid. To me it appeared, that they were movers of both heat and 



Notes, 357 

electricity ; the ratio of the quantity of the latter put in motion, to 
the quantity of the former put in motion, being as the number of 
the series to the superficies. Hence the word electromotor can only 
be applicable, when the caloric becomes evanescent, and electricity 
almost the sole product, as in De Luc's and Zamboni's Columns ; and 
the word calorimotor ought to be used, when electricity becomes 
evanescent, and caloric appears the sole product. 

" The heat evolved by one galvanic pair has been found by the 
experiments which I instituted, to increase in quantity, but to dimin- 
ish in intensity, as the size of the surfaces may be enlarged. A pair 
containing about fifty square feet of each metal, will not fuse platina, 
nor deflagrate iron, however small may be the wire employed ; for 
the heat produced in metallic wires is not improved by a reduction 
in their size beyond a certain point. Yet the metals above men- 
tioned are easily fused or deflagrated by smaller pairs, which would 
have no perceptible influence on masses that might be sensibly 
iguited by larger pairs. These characteristics were fully demon- 
strated, not only by our own apparatus, but by those constructed by 
Messrs. Wetherill and Peale, and which were larger, but less capa- 
ble of exciting intense ignition. Mr. Peale's apparatus contained 
nearly seventy square feet, Mr. WetherilPs nearly one hundred, in 
the form of concentric coils ; yet neither could produce a heat above 
redness on the smallest wires. At my suggestion, Mr. Peale separat- 
ed the two surfaces in his coils into four alternating, constituting 
two galvanic pairs in one recipient. Iron wire was then easily 
burned, and platina fused by it. These facts, together with the 
incapacity of the caloric fluid, extricated by the calorimotor, to per* 
meate charcoal, next to metals the best electrical conductor, must 
sanction the position I assigned to it, as being in the opposite extreme 
from the columns of De Luc and Zamboni. For, as in these the 
phenomena are such as are characteristic of pure electricity, so in 
one very large galvanic pair, they almost exclusively demonstrate the 
agency of pure caloric. 

" When the instrument is lowered into a solution, containing 
about a seventieth of sulphuric acid, a wire, placed between the 
poles, becomes white hot, and takes fire, emitting the most brilliant 
sparks. In the interim an explosion usually gives notice of the 
extrication of hydrogen in a quantity adequate to reach the burning 
wire. Immediately after the explosion, the hydrogen is reproduced 
with less intermixture of air, and rekindles, coruscating from among 
the forty interstices, and passing from one side of the machine to 
the other, in opposite directions and at various times, so that the 
combinations are innumerable. The flame assumes various hues, 



358 Notes. 

from the solution of more or less of the metals, and a froth, appar- 
ently on fire, rolls over the sides of the recipient. When the calo- 
rimotor is withdrawn from the acid solution, the surface of this fluid, 
for many seconds, presents a sheet of fiery foam. 

" I ascertained, that the galvanic fluid, as extricated by this appar- 
atus, does not permeate charcoal. This demonstrates, that it cannot 
be electricity, as of the latter, charcoal is next to metals the best 
conductor." — lire's Chemical Dictionary, Am. Ed., art. Calorimotor. 

2. Deflagrator. 

Figure 75 represents another modification of the voltaic appara- 
tus, invented likewise by Dr. Hare, to which he has given the name 
of deflagrator. It consists of two pairs of troughs, each ten feet long, 
the two of each pair being joined lengthwise, edge to edge, so that 
when the open side of the one A A, containing the plates, is vertical, 
the open side of the other BB, without plates, is horizontal, and 
vice versa. The acid liquor being poured into the trough BB, by a 
partial revolution of the apparatus, it is made to flow into the trough 
containing the plates. Each pair of troughs turns on pivots Z>, D, 
supported by frame work C, C. The pivots are of iron, coated with 
brass or copper, and a communication is made between them, and 
the voltaic series within, by strips of copper. 

The pairs of the series consist of copper cases about seven inches 
long by three inches wide, and half an inch thick, each containing 
a plate of zinc equidistant from the two sides, and kept from touching 
the copper by grooved strips of wood. Each plate of zinc z, is solder- 
ed to the next case of copper on one side, as represented in figure 
75'. The copper cases are open only at the bottom and top, and are 
kept separate from each other by pieces of wood. — American Jour' 
nal of Science and Arts, Vol. vii. p. 347. 



IV. 



Electrical Fishes. 

There are several fishes, which are capable of producing effects 
altogether similar to those of common electricity. 

The gymnotus and the torpedo are the two kinds on which the 
most experiments have been made ; being provided with an organ 
consisting of cells disposed like the metallic plates in a voltaic appar- 
atus, this organ has been considered as the source of the electric 
power. 



Notes. 359 

The effects of the electrical fishes are indisputable ; a few witnessed 
by M. Humboldt may be mentioned. He drove several wild horses 
into a stream which contained many of the gymnoti ; the fishes were 
seen to rise to the surface of the water and crowd under the bellies of 
the horses, which were overcome by the violence of the shocks which 
they received. 

M. Humboldt declares he would not willingly expose, himself to the 
first shocks of a large and irritated gymnotus ; and if by chance a 
person should receive such a shock before the fish is injured or 
fatigued, the pain and swelling are so violent, that it is difficult to form 
an idea of it. " I do not recollect," says he, '• having received by the 
discharge of a Leyden jar a more violent shock than that which I 
received by imprudently placing my feet upon a gymnotus just taken 
from the water. I was afflicted for the rest of the day with a severe 
pain in the knees and almost every other joint." No one acquainted 
with the above facts will be surprised, that the gymnotus should be 
employed in the cure of paralysis. 

The electric action of the gymnotus depends entirely on its voli- 
tion ; it is found on actual trial, that the fish may be repeatedly 
touched, when insulated, or not insulated, without any shock being 
communicated. It even depends on the fish not to act except toward 
the point in which it is most powerfully irritated. 

The action of fishes upon the human organs is transmitted and 
intercepted by the same bodies which transmit and intercept the 
electric current. The smallest spark, however, has never been seen 
to escape from the animal ; irritated in the dark, it does not produce 
the feeblest light, as MM. Humboldt and Bonpland have proved. 

Walsh succeeded in giving the shock to twenty persons, not insu- 
lated, who held each other by the hand ; the person at one extremity 
touching the back, while the one at the other touched the belly of the 
fish. 

MM. deBlainville and Fleurian, of Belleveu, have lately discovered, 
that, when the extremities of the multiplying wire are introduced into 
the electric organ of this fish, the needle undergoes a considerable 
deflection. Dr. Davy succeeded in magnetizing steel needles by this 
power. Thus all the effects of the voltaic apparatus are produced by 
means of the torpedo. 

We will add another fact, observed by M. Gay-Lussac, which is 
very remarkable, if it does not appertain to the feebleness of the ani- 
mal, namely, that when an insulated person touches a torpedo, it is 
necessary that the contact should be -immediate, in order that the 
shock may be felt ; if a key, for instance, be interposed, no effect is 
produced. 



360 



Notes. 



V. 



Declination of the Magnetic Needle at London, from the Time it 
was first observed. 



Years. 



Declination. 



Observers. 



1580 
1622 
1634 
1657 
1672 
1682 
1692 
1722 
1747 
1774 
1786 
1790 
1796 
1800 
1809 
1814 
1815 
1816 
1817 
1818 
1819 
1820 
1821 
1822 
1823 



ILo 15' 
6 



5 

30 
30 

14 20 
17 40 
21 16 
23 17 

23 39 

24 
24 3 
24 11 
24 21 10 
24 17 5 
24 17 54 
24 17 
24 15 43 
24 14 47 
24 11 44 
24 11 18 
24 9 ^ 
24 9 48 



0" East 
— - 





West 





€ 





36 




Barrows. 
Gunter. 
Gellibrand. 
Bo id. 
Halley. 



Graham. 

Cavendish. 
Gilpin. 



Lee. 



We see from this table, that the needle reached its greatest western 
declination in 1814, or 157 years after it was observed by Bond to 
point due north. Since 1814 it has been moving slowly westward. 
If it take as many years to return, as it did to proceed westward, it 
will reach the point of no declination in the year 1971. Should it go 
as far to the eastward as it did westward, and take as long a time, it 
will reach the easternmost declination in the year 2128, and the total 
arc of declination will be 48° 35' 48", the period occupied in travers- 
ing it being 314 years. The average annual variation would be & 
17". But it is much smaller than this towards its western and east- 
ern limits, while it is much greater near the meridian. Thus, during 
the nine years between 1814 and 1823, the progress westward was 



Notes. 



361 



only 11' 22" or V 1.6" annually; while from 1657 to 1672 the varia- 
tion amounted to 2° 30' or 10' annually. Between 1672 and 1682 it 
amounted to 2° or 12' annually. Between 1692 and 1722, the aver- 
age annual increase was 16' 40". This was the maximum. After 
the year 1722 the rate diminished very rapidly. It seems to have 
reached half way or 12° of western declination about the year 1714, 
that is, in 57 years. To complete the other half a hundred years 
were required. These circumstances render it impracticable to cal- 
culate the length of the period of the variation from any data in our 
possession. 



VI. 



Dip or Inclination of the Needle at London. 

The following table exhibits the amount of the dip in London, dur- 
ing an interval of 245 years, according to the best observations which 
have been made. 



Years. 


Dip. 


Observers. 


1576 


71° 30' 


Norman. 


1600 


72 


Gilbert. 


1676 


73 47 


Bond. 


1720 


75 10 


Wiston. 


1723 


75 


Graham. 


1772 


72 19 


Nairne. 


1776 


72 30 


Cavendish. 


1805 


70 21 


Gilpin. 


182 L 


70 3 


Sabine. 



From this table it appears, that the dip reached its maximum in 
London about the year 1720, and that it has been diminishing ever 
since ; but the difficulty of constructing an accurate dipping needle is 
so great, that all the observations hitherto made can be considered 
only as approximations. 



E. fy M. 



46 



362 Notes, 



VII. 



Diurnal Variation of the Magnetic Needle. 

The mean diurnal variation at London, as deduced by Mr. Gilpin 
from 12 years observations, namely, from 1793 to 1805, is as fol- 
lows; March 8',5, June ll',2, July 10',6, September 8',7, December 
3',7. The amount, however, of this change must evidently depend 
upon the strength of magnetism in the needle, the freedom of its mo- 
tions, &c. It may accordingly be increased almost indefinitely by 
diminishing the directive force, and removing, as far as possible, all 
impediments to motion. Thus by reducing the directive force in the 
ratio of 1 to 0,034, by means of two bar magnets, placed in the line 
of the dip, Mr. Christie found a diurnal change in the direction of the 
horizontal needle, amounting to more than 10°. 

The dipping needle is likewise subject to daily variations, espe- 
cially when its directive power is diminished. The following is the 
result of Mr. Barlow's observations relating to this subject. 

" In general, a motion commenced soon after the instrument was 
adjusted in the morning ; but it was not of that gradual progressive 
kind which indicated an uniformly increasing or decreasing power, 
as in the other instrument [horizontal needle]. It passed, for in- 
stance, suddenly from one half or quarter degree to another, more or 
less, and which sometimes in the course of the day would give a 
difference in the dip to the amount of a degree and a half, or even 
more ; but I seldom saw in it a tendency to return ; although when 
I vibrated it toward night, it commonly took up its morning position. 
I made these observations with the needle in various directions, viz. 
with the face of the instrument to the east, west, north, south, &c. ; 
but in every case I obtained the same sort of daily motion. The 
question, therefore, respecting the law of variation of this instrument, 
still remains to be submitted to fixed principles, although there 
can no longer be any doubt, that it is subject to a daily change." 
— Phil. Trans, for 1823, Part II. 



Notes. 363 

VIII. 

Influence of Magnetism on the Rates of Chronometers. 

Mariner's watches or chronometers, 'employed to measure time 
on board of vessels, having in their construction several pieces of 
steel, some of -which are movable, must evidently be subject to 
variation in their rate of going, if placed in the vicinity of magnetic 
bars. This is proved by experiment. Consequently the same effect 
must take place to a certain degree at sea, both on account of the 
continual action of the earth, and the magnetic influence of the fer- 
ruginous masses, by which compass-needles are deflected. For the 
safety of navigation, it is very important to diminish as much as 
possible these changes in the rate of going to which chronometers 
are liable ; and it may undoubtedly be effected, in a great measure, 
by placing them always in the same place, and as far as possible 
from compass-needles and magnetic bars. With this precaution, 
their variation will be very small, and nearly constant ; so that cor- 
rections may be easily applied by means of astronomical observa- 
tions. This important discovery was recently made in England. — 
Edinburgh Phil. Journal, Vol. X. pp. 1 and 342. — Barlow's Mag- 
netic Attractions, 2d ed. 



IX. 



Local Attraction and Barlow\ Correcting Plate. 

" While philosophers were pushing on their inquiries into the laws 
of magnetism, navigators had discovered, • that the compass-needle 
does not continue to point in the same direction as the ship is warp- 
ed round to the different points of the compass,' a simple change 
of position of the ship's head from the north or south, to the east or 
west, producing a change in the variation of the needle of twenty 
or thirty degrees, and varying in amount with every alteration in 
the direction of the ship's head, and every change of position from 
one pole to the other. This effect is obscurely alluded to in Cook's 
* Voyages/ where it appears that it was noticed by Mr. Wales, his 
astronomer. It is also noticed by one or two French navigators, but 
not a hint is thrown out respecting the cause of this anomaly or its 
remedy. The cause was distinctly pointed out by Mr. Downie, the 



364 Notes. 

master of the British ship Glory. ' I am convinced/ said this expe- 
rienced officer, in his report to the Admiralty, published in Walker's 
Treatise on Magnetism in 1794, ' that the quantity and vicinity of 
iron in most ships has an effect in attracting the needle ; for it is 
found, by experience, that it will not point in the same direction 
when placed in different parts of a ship; also it is rarely found that 
two ships steering the same course by their respective compasses, 
will go exactly parallel to each other, yet these compasses when 
compared on board the same ship will agree exactly.' 

" A few years afterwards the influence of the iron in the ship was 
more minutely examined by Captain Flinders, who was the first to 
trace its connexion with the dip, and to show that the effect is dif- 
ferent in quality on opposite sides of the magnetic equator, and 
increases in quantity as the dip in either hemisphere increases. 
With Captain Flinders's observations the matter seems again to have 
fallen into obscurity, till Mr. Bain published his treatise on the Va- 
riations of the Compass, in which the fatal consequences of this 
source of error are so forcibly exposed as to have attracted once 
more the attention of the Admiralty. It fortunately happened, that 
at this period the Arctic expeditions were in contemplation. The 
local attraction of the vessels in these seas was accordingly one of 
the objects to which the attention of the officers was particularly 
directed. The results of the experiments made in these instances 
are detailed by Captains Ross and Parry in the accounts of their re- 
spective voyages. The amount of the disturbing force was found 
to be such as to call for the most prompt and efficient remedy, the 
difference of the bearing of an object having, on one occasion, 4>een 
found by Captain Sabine to be 50°, merely by a change of position 
of the ship's head from east to west. 

" It may appear surprising, that an error of such amount should 
have so long escaped the observation of navigators. But the fact is, 
that, owing to the changes which have taken place in ship-building, 
this error was much smaller formerly than it is now. It is only within 
a few years that pig-iron has been employed for ballast, the weight of 
which, in some vessels, exceeds three hundred tons. An immense 
surface of iron is also introduced by the admirable invention of iron 
tanks to supply the place of the old water-casks. Moreover, the 
knees, sleepers, and sometimes even the riders, are now of iron ; 
and some attempt has recently been made to employ gun-carriages 
of the same material. But of all innovations of this kind, the inven- 
tion of the patent capstan by Captain Phillips, is that which, from 
its form and situation, has the greatest effect on the compass. So 
powerful, indeed, is its action, that, without the means afforded by 



Notes. 365 

Professor Barlow's correcting plate, its use would have been pro- 
hibited in all vessels of a smaller class than frigates. In the Griper, 
for instance, the local attraction was 14° at east and west, making 
an extreme difference in the River Thames of 28°, which was re- 
duced to 16°, by the removal of the capstan. 

" This statement shows sufficiently, that the errors which have be- 
come so great in consequence of the introduction of masses of iron 
into the structure and equipment of ships, might easily escape the 
attention of navigators at a time when, these causes existing only to 
a small extent, the errors were comparatively imperceptible. 

" The object which appears to have suggested and directed Pro- 
fessor Barlow's inquiries, was those errors in the needle which we 
have been describing. To discover a practical remedy for them 
was the scope of his design. In the pursuit of his object he appears 
to have been singularly happy, and to have conducted his experi- 
ments with great discretion and considerable sagacity. He pro- 
cured a solid iron ball thirteen inches in diameter, and weighing 
two hundred and eighteen pounds. When the compass was above 
the ball, he found, that the north end of the needle was attracted to- 
ward it, and that when it was below the ball the south end was 
attracted toward it, and that in traversing the interval between 
these two positions, it always passed through a point in which the 
ball had no effect on the needle. In Professor Barlow's apparatus the 
compass was fixed, and the ball was suspended by a pulley, so adjust- 
ed, that the ball might be moved upwards and downwards at pleasure. 
This circumstance is not material to our statement, and we merely 
notice it in passing, to prevent any misconception. The question 
would naturally be suggested — Are all these points in which the ball 
exerts no influence on the needle situated in the same plane? — 
if so, is this plane parallel or inclined to the horizon 1 A series of 
experiments directed to this point showed, that they were all in the 
same plane, and that the plane is inclined to the horizon, dipping 
toward the south and making an angle with the horizon equal to the 
complement of the dip. When the needle had its natural dipping 
position, this result might have been naturally anticipated, for then 
the ball would be symmetrically situated in respect to the needle ; 
but that the effect would still be the same when the needle was hori- 
zontal, was a discovery as new as it was important. 

" Having traced this circle on the sphere, he assumed the plane of 
it as an equator, and the direction of the dipping needle as a princi- 
pal axis. This plane, which Professor Barlow calls the plane of no 
attraction, implies a misconception into which he had probably been 
led by the apparent result of his experiments, and the want of a 



366 Notes. 

rigorous and comprehensive analysis to supply the deficiencies of 
mere observation. It were needless to repeat, that the force of 
attraction does not vanish in this plane. This plane will intersect 
the horizon in an east and west line. Here, then, he had a most 
obvious and simple method of assigning and fixing the position of the 
compass relatively to the ball, by latitudes and longitudes referred 
to the plane and one of these east or west points. It was natural to 
suppose, that the formulas which define the law of deviation would 
be more simply expressed by latitudes, reckoned from this magnetic 
equator, and longitudes reckoned from one of these points in which 
the magnetic equator intersected the horizon. Professor Barlow 
appears to have assumed the zero of longitude in the west point. 
Now, if the deviation of the compass, due to the action of the iron 
ball, be expressible in any formula, this formula can only depend on, 
or be a function of, the magnitude of the ball, the distance of its 
centre from the point of the compass, and the latitude and longitude 
of the compass. In the first series of experiments the same ball 
was constantly employed, and it was kept at the same distance from 
the compass. For these experiments the magnitude and distance 
of the ball would be constant, and therefore any consideration of 
them was unnecessary, — the deviation could only vary by a change 
of latitude and longitude. But for the sake of still further simplify- 
ing the experiments, it was desirable to consider separately the 
effects of the latitude and longitude. This was easily done, for by 
causing the compass to move over the same meridian, the lati- 
tudes would have every change from zero to 90°, both north and 
south, while the longitude would be constant. Again, by causing 
the compass to move over a small circle, parallel to the equator, the 
longitude would have every value from zero to 360°, while the lati- 
tude was constant. By pursuing the course suggested by these ob- 
servations, the effects of the latitude and longitude were separately 
obtained, unmixed with the effects due to the changes of the other 
quantities. The result of his experiments may be stated as follows ; 

I. By moving the compass over the meridian whose longitude was 

r j lL . sin. lat. X cos. lat. r ,. 

zero, it was found that — : lj.,. was constant ; so far there- 

tang. deviation 

fore as the deviation (/I) depended on the latitude (;.), 

tang. A = m sin. 2 X . . . (1). 

II. By moving the compass over a small circle parallel to the 

equator, it was found that '. — te . . — was constant; as far there- 

tang. deviation 

fore as the deviation depended on the longitude (7), 

tang. J z=z n cos. I . . . (2). 



Notes. 367 

" This was not precisely the course which Professor Barlow fol- 
lowed. For the purpose of computation, it was found to be more con- 
venient to move the compass over a circle in which both latitude 
and longitude varied ; but as the law of the deviation in respect to 
the latitude was already known, this did not much complicate the 
inquiry ; it was found, of course, that 

5m. 2 X cos. I . . , , 

= p constant . . . (3). 

tang. 4 ;. , 

III. Having ascertained the law of deviation as it depends on the 
latitude and longitude of the compass, the next point was to deter- 
mine how, other things remaining constant, the deviation would 
vary with a change of distance, and it was found, that 

tmg - J = is&j*-- " ' ' (4) ' '• 

In these formulas, m, n, p, and q are constant. 

IV. The last result might have been determined almost without 
any experiment; but another discovery awaited this part of the 
investigation, as important as it was unexpected. On repeating the 
experiments with iron shells, Professor Barlow found for balls of 
the same dimensions the same results, whether they were solid or 
hollow, provided their thickness exceeded about 2V of an inch. It 
was shown by experiment, that a she.ll of iron plate of ^ inch in 
thickness produced about two-thirds of the effect of a solid ball, and 
in general a series of experiments directed to this point led to the 
conclusion, that a certain thickness, exceeding ^ of an inch, was 
necessary to the full developement of the magnetic action. This 
last result was so singular, that it could not be expected, that philoso- 
phers would admit it without rigorous verification. For this pur- 
pose Captain Kater executed a series of experiments with three 
cylinders of soft iron, having the same external dimensions ; one made 
of sheet-iron, one of chest plate, the third being solid ; and he com- 
pletely confirmed the previous deductions of Professor Barlow. 

" These formulas and laws were in the first instance purely empir- 
ical. The author has, however, in the last edition of his Essay on 
Magnetism, delivered an analysis from which he has drawn the 
same results. This part of his labors we think of the less value, 
that it proceeds upon an hypothesis, which not only blinks all the 
difficulties of a rigorous analytical investigation, but is so partial as 
to leave untouched many of the points most important to be deter- 
mined. The principles on which he founds his calculations are 
these ; 1. With Coulomb he assumes, that the law of attraction varies 
inversely as the square of the distance. 2. That all the phenomena 
of terrestrial magnetism and iron magnetized by induction, may be 



36 S Notes. 

referred to two poles indefinitely near each other in their general 
centre of attraction. This is a step beyond what a complete and 
rigorous resolution of the problem would permit us to assume. 
3. That the magnetic fluid suffers only insensible displacements. 
This is, as far as we can perceive, a dormant principle in Professor 
Barlow's investigation, although it is the very essence of the physical 
conditions of the problem. 4. That the action of the magnetic fluid 
is confined to the surface of the body. This is the objectionable 
part of his hypothesis, — an assumption which reduces the value of 
his analytical investigations to zero. He supposes, that it may be 
inferred from his experiment with iron shells ; but he is quite mis- 
taken in this matter. The accumulated effect of the action of all 
the particles of magnetic fluid, may be the same as would be pro- 
duced by a magnetic fluid diffused over the surface. The attractive 
and repulsive virtues may so balance each other as to produce this 
effect ; but the analysis which assumes, that, therefore, the magnetic 
fluid is confined to the surface, must needs be very unsatisfactory. 
For these reasons, while we willingly bestow our meed of approba- 
tion for his successful experiments, we must be permitted to think 
that his analytical calculations are of little value, and quite as likely 
to mislead, as to direct, the course of our inquiries. 

" These experiments suggested at once a remedy for the errors 
due to the local attraction of ships ; for the action of any mass of 
iron may be referred to two points indefinitely near each other in 
the general centre of attraction of the masses of iron on board. If, 
therefore, in the line joining this centre and the needle, we place on 
the opposite side a mass of iron, whose action on the needle shall 
be just equal to that of the disturbing force of the vessel, these 
forces being opposite will destroy each other, and leave the needle 
at liberty to obey the action of the earth's magnetism. Experiment 
soon showed, that a small plate of iron placed within a few inches of 
the compass was sufficient to produce this effect.* This was Profes- 
sor Barlow's first suggestion to the Admiralty. 

" The first experiments with the correcting plate were made on 
board his majesty's ship Leven, which sailed under the command of 
Captain Bartholomew, in 1820, to the western coast of Africa, but 
returned the following year under the command of Captain Baldey 



* The correcting plate consists (fig. 191) of two circular discs of iron, about 14 
inches in diameter, and of such a thickness that a square foot would weigh 
about 3 pounds, supported on a copper rod AB, about an inch and a half in di- 
ameter. The discs arc separated by a sheet of pasteboard, and pressed together 
by screws. 



Notes. 369 

in consequence of the death of the former officer. A very extensive 
series of observations led to the most satisfactory results. 

" It was obvious, indeed; without any such practical determination, 
that this must have been the case ; but still, from that distrust with 
which practical men always regard the discoveries of abstract inves- 
tigation, this remedy couki only be classed with the dreams of theo- 
rists till confirmed by actual experiment. Two cases of a decided 
character had occurred very recently, which seemed to furnish an 
experimcntum cruris, and on these it was resolved to try the opera- 
tion of the correcting plate. 

" Captain Flinders had observed, that, with an equal north and south 
dip, he found an equal quantity of deviation, but in a contrary direc- 
tion. To see whether the plate would meet these circumstances 
was the point left for the decision of Captain Basil Hall, in his voy- 
age in the Conway round Cape Horn to the western coast of America. 
Observations were accordingly carried on from England below Cape 
Horn to the latitude of 61 0, south, and throughout this great arc of 
terrestrial latitude the results are the most satisfactory that can be 
desired. 

•• The next point to be settled was this. It had been ascertained, 
by the observations of Captains Ross and Parry, that the effect pro- 
duced by the iron of the ship had increased with immense rapidity 
in approaching towards the pole. Would the power of the plate 
increase with rapidity 1 It seems to us that not a shadow of doubt 
could have been rationally entertained ; but, to make ' assurance 
doubly sure,' Lieutenant Foster, who had already received the 
thanks of the Board of Longitude, for his experiments on this and 
other scientific subjects in the Conway, was now appointed to the 
Griper, which was about to leave England for Spitzbergen, under the 
command of Captain Clavering. His experiments were the more in- 
teresting, that they were made in very high latitudes, where hitherto 
the compass had been generally stowed away as useless, both on this 
account as well as from the circumstance of the ship's local attraction 
being much greater than usual. By observations made while the 
vessel was lying at the Nore, the bearing of an object was found 
to differ 28° with the ship's head at east and west. That is, the 
local attraction was 14° at each of these points, and proportionally 
great in all intermediate positions, an amount of deviation truly 
astonishing, and which Captain Clavering ascribed to the influence 
of the spindle of the patent capstan, a suggestion which was verifi- 
ed by experiment on the return of the vessel, as we have already 
stated. To counteract this strong power it was necessary to bring 
the iron plate which was 14 inches in diameter, to a distance from 

E. Sf M. 47 



370 



Notes. 



the middle of the pedestal of 7J inches, and the centre of it 7£ 
inches below the pivot of the needle, in which situation abaft the 
compass, it balanced the local attraction of the ship and left it free 
to obey the natural directive power of the earth ; this was proved 
by taking the variations of the compass with and without the plate 
from England to the North Cape, when the close agreement of the 
former and the great discrepancy in the latter were so marked, that 
the vessel was navigated during the remainder of the voyage alto- 
gether by the corrected compass. 

" The Griper was swung at three different ports during the voyage ; 
at Drontheim, Hammerfest, and Spitzbergen, and the local attrac- 
tion ascertained at every station, first with and then without the 
plate. With the plate the deviations were reduced to quantities 
very little exceeding what might be attributed to errors of observa- 
tion ; without the plate they were found to be at the east and west, 
or maximum points, as follows ; 

Nore . . . . 14° 00' 
Drontheim . . . 21° 23' 

Hammerfest . . . 24° W 
Spitzbergen . . . 34° 4% 

" The nature, however, of these irregularities, and the importance 
of Professor Barlow's plate, will be more distinctly seen from the 
following table of variations with and without the plate, taken during 
the voyage. 









Variation 


Variation 


Time oi Obser- 


Latitude. 


Longitude. 


Ship's Head. 


without the plate. 


with the plate. 


vation. 


65° 6' N 


6° 54' E 


N 


( 26° 1' W 

\ 11 28 


i 24° 23' W 
\ 25 2 


May 18, 1823 


Ditto 


Ditto 


NE 


do. do. 


66 57 


7 20 


N 


24 52 


25 30 


May 20, do. 


66 15 


8 


E*N • 


2 14 


21 15 


do. do. 


66 35 


9 12 


NEfE 


11 58 


22 43 


May 21, do. 


67 21 


9 4 


NE£E 


( 18 4 
\43 5 


22 12 


May 23, do. 


Ditto 


Ditto 


W 


20 


do. do. 


69 8 


14 30 


NE 


/ 13 35 
V 40 37 


13 35 


May 28, do. 


Ditto 


Ditto 


W 


14 28 


do. do. 



" The uniformity of the change in the variation when the correct- 
ing plate was employed is obvious at a single glance ; whereas the 
rapid and large irregularities which are shown, when the plate was 
not used, placed in the strongest light its great importance. Thus 
we see on the 18th May, by simply warping the ship round from N. 
to NE. the variation experienced a change of 15° ; on the 20th, by 
a change from N. to E. a N., the variation was reduced from 24° 52' 
to 2° 14'-; and lastly, on the 28th, the change of direction in the 
ship's head from N. E. to W. produced an increase of nearly 30° in 
the variation. These are not solitary instances. The log-book pre- 



Notes. 371 

sents a continued succession of them. Under such circumstances, it is 
obvious, that the compass becomes a mere piece of useless furniture." 

"Every reader," says Professor Barlow, M whether a nautical man 
or not, must be aware of the great amount of error and fatal conse- 
quences which might arise in a few hours to a vessel in the channel, 
in a dark and blowing night, having for its only guide a compass 
subject to an error of 14° in opposite directions at east and west, 
the very courses on which she would be endeavouring to steer ; and 
who can say how many of the mysterious wrecks which have taken 
place in the channel are to be attributed to this source of error, of 
which the most recent, that of the Thames, Indiaman, is a serious 
example. This vessel, besides the usual materials, guns, &c, had a 
cargo of more than 400 tons of iron and steel ; and it may be easily 
imagined, that such a cargo would produce an effect on the compass 
at least equal to that of the Griper and Barracouta ; and this alone 
would be quite sufficient to account for the otherwise inexplicable 
circumstance, that, after having Beachy Head in sight at six o'clock in 
the evening, the vessel should have been wrecked upon the same 
spot at one or two in the morning, without the least apprehension of 
being at all near shore." — See Barlow's Magnetic Attractions , 2e? ed. 

[In the above note, the compiler has availed himself of an abstract 
and some remarks contained in the Westminister Review^ for April, 
1825.1 



Theory of Magnetism, by M. Poisson. See Ann ales de Chimie, 
pour Fevrier, 1824. 

" The first step in this inquiry was obviously to reduce to three 
rectangular co-ordinates the results of all the attractions and repul- 
sions exerted by the magnetic elements of a magnetized body of any 
imaginable form upon a given point, situated either within or with- 
out the body. By adding to these results, as belonging to any point 
within the system, those of the external magnetic forces that act 
upon the body, we have the whole forces that tend to separate the 
two fluids which are united at the point in question. And if the 
matter of the body opposes no resistance to the displacement of the 
two fluids ; or in other words, if there be no coercive force, it will 
be necessary, in order that there may be an equilibrium, that all 
the attractions and repulsions should destroy each other. The sum 
of the forces, therefore, in the directions of these three co-ordinates 
are severally made equal to zero. These equations are at first, as 
might be expected, somewhat complicated ; but, by means of certain 
transformations, the triple integrals, in terms of which they are ex- 



372 JYotes. 

pressed, are reduced to double integrals, and the equations very con- 
siderably simplified. From these equations M. Poisson has been 
able to deduce the following general principles, remarkable for their 
singular simplicity, novelty, and beauty. 

" I. That, notwithstanding the boreal and austral fluids are dis- 
tributed throughout the mass of a body, magnetized by induction, 
the attraction and repulsion, which it exerts externally, are the 
same as if it were merely covered by a very thin stratum, formed of the 
two fluids in equal quantities, and such that their total action upon 
all the points within them should be equal to nothing. This theorem 
extends to all bodies whatever. 

" II. When the general formulas of this memoir are applied to a 
hollow sphere of uniform thickness, the following remarkable result 
is obtained ; — 'A magnetic needle, placed within a hollow sphere of 
soft iron, and so small as not to exert any sensible influence on the 
sphere, will not be subject to any magnetic action, and will consequent- 
ly not exhibit any polarity, from the effect of the earth's magnetism, 
or from that of any other magnet placed without a hollow sphere.' 
We need not stop to point out the striking analogy between this 
result and the case of a material particle, placed within a hollow 
shell of matter, attracting according to the general law of gravitation. 

" III. If the general formulas be applied to the particular case of a 
sphere magnetized by the action of the earth, they, admit of being 
integrated in finite terms, and of being completely resolved. We 
are, therefore, enabled to determine every thing relative either to 
the direction of the line of polarity, or the intensity of the magnet- 
ism in the solid part of the sphere, or its action on any point with- 
out, given in position. In this case, although the magnetism is not 
confined to the exterior surface of the hollow sphere, and although 
its intensity may be determined for any point of the hollow shell, yet 
the magnitude of the three component forces, produced by it, is wholly 
independent of the thickness of the metal, — it is determined simply by 
the radius of the external surface and the co-ordinates of the point 
on which the forces act. When the distance of this point from the 
centre of the sphere is very great compared with the radius, each of 
the three forces is very nearly proportional to the cube of the radius 
directly, and the cube of the distance inversely. These forces may 
be reduced to two, a force to, or from, the centre of the sphere, and 
a force in the direction of the dipping needle. The former of these 
vanishes when the point is situated in the plane passing through the - 
centre of the sphere and perpendicular to the direction of the latter 
force. Hence, if a small magnetic needle be placed in this plane, 
the direction which it would assume by virtue of the action of the 
earth will not be altered by the attraction of the sphere. We must 



Notes. 373 

not, however, infer, that the attraction vanishes in this plane ; for 
the second force does not vanish at the same time with the first ; it 
will be subtracted from the first, and its effect will be to retard 
more and more the oscillations of the needle, as it is brought nearer 
the surface of the sphere. At the surface itself, and in any plane 
intersecting it, this force is equal and contrary to the actiou of the 
earth ; so that in this situation the small needle will only be urged 
in the direction of the radius ; and provided it were so small that its 
action on the sphere would be inconsiderable, in the plane perpen- 
dicular to the dipping needle, and very near the surface of the 
sphere, the needle would be exempt from all magnetic action, and 
would have no determinate position. 

" M- Poisson has announced his intention to investigate in a second 
memoir the laws which regulate the distribution of magnetism in 
needles of steel magnetized to saturation, and in needles of iron 
magnetized by induction, by means of the general formulas which 
have been demonstrated ; and from these distributions to deduce the 
phenomena of their mutual attraction and repulsion." 

XL 

Effects of Temperature on the Magnetic Forces. 

" In a very able paper on this subject, Mr. Christie has given the 
following results ; 

1. From 3° of Fahrenheit, and even much lower, up to 127°, the 
intensity of the magnets decreased, as the temperature increased. 

2. With a certain increment of temperature the decrement of 
intensity is not constant at all temperatures, but increases as the tem? 
perature increases. 

3. From a temperature of about 80°, the intensity decreases very 
rapidly as the temperature increases, so that if, up to this tempera- 
ture, the differences of the decrements are nearly constant, beyond 
that temperature the differences of the decrements also increase. 

4. Beyond the temperature of 100°, a portion of the power of the 
magnet is permanently destroyed. 

5. On a change of temperature, the greatest portion of the effect 
on the intensity of the magnet is produced instantaneously, which 
proves that the magnetic power resides on or very near the surface. 

6. The effects produced on unpolarized iron, by changes of tem- 
perature, are directly the reverse of those produced on a magnet, an 
increase of temperature causing an increase in the magnetic power 
of the iron, the limits between which Mr. Christie observed, and they 
were 50° and 100°." — Phil Trans, for 1824, Part 2d. 



374 Notes. 

XII. 

Diurnal Variation of the Terrestrial Magnetic Intensity. 

" The following interesting table, given by Mr. Christie in the 
paper above referred to, shows the diurnal variation of the magnetic 
intensity in May and June, according to his own observations, and 
those of Hansteen's ; 



Intensity according to Hansteen's 
Observations in 1820. 


Intensity according to Mr. Christie's 


Observations in 1823 




Hour. May. June. 


Hour. 




May. 


June. 


8h O'a.m. 1,00034 1,00010 


7" 30' A. 


M. 


1,00114 


1,00061 


30 1,00000 1,00000 


10 30 




1,00000 


1,00000 


4 p.m. 1,00299 1,00251 


4 30 




1,00175 


1,00223 


7 1,00294 1,00302 


7 30 




1,00220 


1,00239 


30 1,00191 1,00267 


9 30 




1.00231 


1,00209" 



Edinburgh Journal of Science, for July, 1825. 

xm. 

Magnetism in Motion. 

" We are indebted to M. Arago for the discovery of this new 
kind of magnetic action. 

" The new phenomena thus made kuown are of two kinds ; the first 
relate to the influence of bodies at rest upon a magnetic needle in 
motion ; the second, the influence of bodies in motion upon the mag- 
netic needle at rest. 

" 1. A magnetic needle, suspended by a vertical thread and de- 
flected 53° from the magnetic meridian, returns to it by a series of 
oscillations, gradually diminishing. If we note the number of oscilla- 
tions that take place before it is reduced to an arc of 43°, we shall 
find that the number varies with the nature of the substance above 
which the needle oscillates. 

" The results furnished by the metals, especially by copper, are 
very remarkable. A plate of this metal was found to reduce to four 
the number of appreciable oscillations of a needle, which in the air, 
and without the influence of copper, made more than 400 oscillations. 

" It is well ascertained by experiment, that a magnetic needle ex- 
periences from all bodies, and especially from metals, an influence 
which diminishes rapidly the extent of the oscillations without alter- 
ing their duration. 

" 2. M. Arago, guided by a principle in mechanics, that action and 
reaction are equal, succeeded in drawing a needle at rest by a plate 
in motion. 



Notes. 375 

"The apparatus employed in these new experiments is composed of 
two parts, separated from each other ; the first consists of a kind of 
clock-work, all the wheels of which are of copper, which communi- 
cates to a plate a motion of rotation, regulated by a fly, the velocity of 
which is measured by a hand, that points out the number of revolu- 
tions performed in a given time. The second part consists of a glass 
cylinder, closed at the lower end by a sheet of paper and at the upper 
by a glass plate, at the centre of which is fixed a copper rod, which is 
raised or depressed at pleasure, and which carries the thread to 
which is suspended a magnetic needle. An index shows the direc- 
tion of the needle, the centre of which is made to coincide with the 
centre of the turning plate. 

" A moderate velocity of rotation, impressed upon the metallic plate, 
causes the needle to deviate in a remarkable manner. If the motion 
is gentle and uniform, the needle fixes itself in a determinate posi- 
tion ; if the motion is rapid enough to produce a deviation of more 
than a right angle, the needle is drawn on and made to describe an 
entire circumference, containing its motion with a velocity that goes on 
increasing till the plate ceases to turn. To give an idea of the in- 
tensity of the force under consideration, we will state the following 
result. 

"A plate of copper about a twelfth of an inch thick, moving with a ve- 
locity of four or five revolutions in a second, impressed, at the distance 
of more than an inch, a motion of rotation upon a magnetic bar, the 
length of which was a little less than the diameter of the revolving disc. 

" M. Arago wishing to have a datum upon the magnetic power of 
copper destitute of motion, subjected a needle to the action of a bar 
of this metal, and observed, at the distance of TT j%^ of an inch, an an- 
gular displacement of above 2'. 

"All the metals are capable of producing similar phenomena, but to 
a much less degree than copper. If we substitute for the plate the same 
metal, reduced to powder or shavings, the needle is scarcely affected. 

" This new force diminishes with the distance, and it is inferred, 
from the phenomena of rotation, that it acts perpendicularly to the 
radii of the disc and parallel to its surface. 

" The following experiment shows, that there exists also a repulsive 
force perpendicular to the surface of the disc. If we suspend verti- 
cally a magnetic needle by one of its extremities to the end of a scale- 
beam, and balance it, upon making a copper disc revolve below the 
needle it is repelled, since the other end of the balance preponderates. 

" M. Arago has further proved, that there exists a force acting in 
the direction of the radii and parallel to the surface of the plate. 
For, if we render vertical a dipping needle, and cause the plane of its 
motion to pass through the centre of the plate, upon bringing the point of 



376 Notes. 

the needle above the different points of the same radius or its prolong- 
ation, it will be seen that it is repelled, when it falls without the plate. 

" This repulsive force diminishes in proportion as we approach 
the centre, till it becomes nothing at a point nearer the edge than the 
centre, after which it changes to an attractive force; finally, it is re- 
duced to nothing at the centre. 

" Thus the action of a circular, horizontal, metallic plate, turning 
on its centre, may be decomposed into three forces; one perpendicu- 
lar to the plate, another horizontal and perpendicular to a vertical 
plane, passing through the radius that terminates at the projection of 
the pole of the needle, and a third directed parallel to the same radius. 

" M. Arago sought the relation of these three forces, and found 
that it varies with the velocity of the rotation of the plate. The 
importance of these new facts is so much the greater as the connex- 
ion with former facts is less known. 

" If we make narrow slits in the disc, at a small distance from the 
centre, there is very little deviation produced. 

" More curious experiments of M. Arago have been repeated and 
confirmed by other philosophers. Herschell and Babbage have 
moreover remarked, that, by filling up the slits made in the disc by 
a metal, the magnetic influence of which is much inferior, we restore 
to the plate apparently all its energy; but, if we make use of a powder 
or a liquid in closing the slits, we do not repair the loss of intensity 
occasioned by the interruption of continuity. 

u The same philosophers have represented by the following num- 
bers the powers of different metals. Copper 1 ; zinc 0,93; tin 0, 46; 
lead 0, 25 ; antimony 0, 09 ; bismuth 0, 02. They have also deter- 
mined, that a screen of any substance, excepting iron, nickel, cobalt, 
manganese, exerts no influence when it is placed between the mag- 
net and the revolving plate, and that a turning plate does not draw 
after it another plate, left at rest. 

" Professor Barlow has observed, that motion increases the magnetic 
power of iron. 

" M. Ampere put in motion by the action of a revolving disc a voltaic 
conductor, arranged in the form of a spiral at each of its extremities. 

" The theory of the influence of revolving plates is not yet perfect- 
ed. It has been supposed, that each pole of the needle gives rise to a 
pole of a contrary name, which is displaced in the surface of the plate, 
and which disappears less rapidly than it is formed. But the con- 
sequences of this hypothesis are not in exact accordance with facts, 
especially that relating to the experiment with the balance." 

Despretz's Traite Elementaire de Physique, p. 538, note. 

THE END. 



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